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Lecture Comments (36)

1 answer

Last reply by: Professor Hovasapian
Thu Oct 13, 2016 6:16 AM

Post by Parth Shorey on October 5 at 03:58:25 PM

How did you get the density of lactic acid?

1 answer

Last reply by: sandi imayeguahi
Mon Jun 6, 2016 2:25 PM

Post by sandi imayeguahi on June 6 at 02:19:24 PM

how did you get the mass of malic acid at 37:23?

2 answers

Last reply by: Professor Hovasapian
Thu May 19, 2016 3:55 PM

Post by Michael Amin on May 18 at 01:49:24 PM


I was wondering what book can I use to supplement your lectures for biochemistry. Would appreciate your response!


1 answer

Last reply by: Professor Hovasapian
Sun Feb 7, 2016 11:40 PM

Post by Mohamed E Sowaileh on February 7 at 05:21:27 AM

Professor Hovasapian,

From your respond to Mr. Apolonia, I understood that in order to have a full, solid comprehension in biochemistry we have to study physical chemistry, and without physical chemistry we may understand biochemistry but we may not have a very solid comprehension. Is that right ? Please correct me.

A pharmacy student.  

2 answers

Last reply by: Apolonia Gardner
Tue Nov 24, 2015 1:17 PM

Post by Apolonia Gardner on November 24, 2015


I am a high school senior about to send off my applications for college. I am stuck on one thing – my intended major. Biology and chemistry have been my favorite courses throughout high school, and I would like to get a college degree that will enable me to perform research with viruses. My lifetime goal is to find a cure for a disease. From your experience, what undergraduate major should I shoot for? Biochemistry? Microbiology? Molecular Biology? Immunology? Chemical Biology? Organic Chemistry? Pharmaceutical Science? Any guidance is appreciated.

3 answers

Last reply by: Lilly Anne
Wed Sep 2, 2015 3:26 PM

Post by Lilly Anne on September 1, 2015

Hello Professor,

I am taking a Biochemistry I course this semester at my university. I took Organic Chemistry years ago and i don't have as much memory of it. I am worried that i won't succeed in Biochemistry because i don't have a recent excellent understanding of biochemistry.

What topics in organic chemistry would you recommend i review and study in order to succeed in Biochemistry I?

Thank You

Thank You

0 answers

Post by taoheed kasumu on May 9, 2015

is there a possibility that your slides can be typed and presented like the organic chemistry tutor? I feel like it makes it much faster to present the material!

1 answer

Last reply by: Professor Hovasapian
Fri Feb 27, 2015 1:31 AM

Post by Robert Bright on February 26, 2015

i love your hair!

1 answer

Last reply by: Professor Hovasapian
Sun Feb 1, 2015 6:19 PM

Post by Okwudili Ezeh on January 31, 2015

How come there are no lessons on DNA replication and transcription?

2 answers

Last reply by: Zachary McCoy
Sat Nov 1, 2014 6:11 PM

Post by Crystal Rosenbrook on September 18, 2014

Is there any way to increase the playback speed of the videos?

2 answers

Last reply by: James Plumb
Wed Aug 13, 2014 2:21 AM

Post by James Plumb on August 8, 2014

This questioned doesn't have to due with this lecture, but my question is if you had a more concrete date on the release of physical chemistry? Thank you.

1 answer

Last reply by: Professor Hovasapian
Tue Nov 5, 2013 2:37 PM

Post by robina saeed on November 5, 2013

Hello Professor,

I am starting Biochemistry this month. I am done with General Chemistry. I have no Organic Chemistry.  Do I need Biology to do really well in Biochemistry?

Take Care,
Robina Saeed

1 answer

Last reply by: Professor Hovasapian
Fri Aug 2, 2013 2:49 PM

Post by robina saeed on August 2, 2013

Hi Professor
Thanks for the earlier response. I have one more question. I have had a full year of general chemistry.  Do I need organic chemistry to begin this course?  Never had organic chemistry.


1 answer

Last reply by: Professor Hovasapian
Sun Jul 28, 2013 11:52 PM

Post by robina saeed on July 28, 2013

Hi Professor
What course work do you recommend I review before starting this course?  General Chemistry or Organic Chemistry?

0 answers

Post by Leili Reza on March 4, 2013


1 answer

Last reply by: Professor Hovasapian
Mon Feb 18, 2013 1:28 AM

Post by Basil K on February 15, 2013

Really excited to start on these lectures! You are an excellent teacher Professor Hovasapian!

Related Articles:

Aqueous Solutions & Concentration

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Aqueous Solutions and Concentration 0:46
    • Definition of Solution
    • Example: Sugar Dissolved in Water
    • Example: Salt Dissolved in Water
    • A Solute Does Not Have to Be a Solid
    • A Solvent Does Not Have to Be a Liquid
    • Covalent Compounds
    • Ionic Compounds
    • Example: Table Sugar
    • Example: MgCl₂
    • Expressing Concentration: Molarity
  • Example 1 14:47
    • Example 1: Question
    • Example 1: Solution
    • Another Way to Express Concentration
  • Example 2 24:00
    • Example 2: Question
    • Example 2: Solution
    • Some Other Ways of Expressing Concentration
  • Example 3 29:30
    • Example 3: Question
    • Example 3: Solution

Transcription: Aqueous Solutions & Concentration

Hello and welcome to and welcome to the first lesson of Biochemistry.0000

Biochemistry is absolutely an extraordinary, extraordinary class.0006

There is a lot of information and all of the information is absolutely exciting.0012

Before we actually jump into the biochemistry with proteins and lipids and carbohydrates and metabolism, what we want to do is do just a little bit of a general chemistry review all of the things that are going to be very, very important.0018

Now, you've seen most of these things before but it may have been a while since you've actually worked with them.0031

For the first couple of lessons, we're just going to do a nice chemistry review just to get everything going again and then we'll jump into the biochemistry.0035

Let's get started and welcome again.0043

OK. We're going to start off by discussing aqueous solutions and the notion of concentrations.0048

An aqueous solution, the reason we're discussing this is because the body of chemistry that takes place in the body, takes place in an aqueous solution - in water; we're mostly just water.0055

And it's just a whole bunch of molecules that are dissolved in that water running into each other and doing the things that they do.0070

Because this is biochemistry, the chemistry of biological systems, the chemistry of biological molecules, all of the chemistry that you learned in general chemistry regarding aqueous solutions, all of it absolutely applies here.0073

Let's go ahead and start with our definition of solution and we'll work our way forward.0089

Solution is just a solute dissolved in a solvent.0099

There are two things, you have the solvent and then you have the solute, the thing that's actually dissolved in it.0113

For biochemistry, it works out really, really great because the only solvent that we're concerned with is water, thus, aqueous.0119

If you remember from organic chemistry, you're going to have all kinds of solvents.0124

You can have hexane, you can have ethyl acetate, you can have all kinds of things alcohol, but for biochemistry, its water, so it makes our life that much simpler.0130

Some examples of solutions... Let's see what we've got.0140

We have sugar dissolved in water. That is a sugar solution, sugar dissolved in water.0145

The sugar is the solute and sure enough, the water is the solvent.0155

H2O is the solvent.0164

In this particular case, you have a solid, the sugar crystals, and the solvent happens to be a liquid.0167

Now, it doesn't have to be this way.0172

A solute does not have to be a solid and a solvent doesn't have to be a liquid.0174

It just turns out that way most of the time and certainly in our case.0179

Let's see. How about another example? Let's take salt dissolved in water so a salt solution.0185

When a salt is dissolved in water, again, the salt is your solute and H2O is again, the solvent.0194

Oops, not solute. What we have is solvent. OK.0209

Now, as I said before, a solute does not have to be a solid.0218

A solute does not have to be a solid.0223

It just happens to be most of our experience from general chemistry, organic chemistry and just normal day to day stuff.0233

Most solutes happen to be solid because we dissolve them in something.0240

We see the crystals just sort of disappear as the solution is created but it doesn’t have to be that way and in fact we have a daily example of that- carbonated water.0244

Carbonated water or soda is actually just CO2 gas dissolved in water so a solute is a gas.0255

Let me write this out... carbonated water.0265

In this particular case, the CO2 is the solute.0271

It is a gas and H2O is the solvent. H2O is the solvent.0277

Now, in this particular case, in order to make sure that the CO2 actually stays dissolved in the water to create the carbonic acid solution - the carbon dioxide solution, we have to put it under pressure.0284

Which is why when you pop the can, the carbon dioxide escapes and that's the bubbles rising. Ok.0295

Now the solvent itself... the solvent does not have to be a liquid.0304

Does not have to be a liquid...0316

This notion of a solution is actually a very, very broad definition.0321

It's when something is dissolved in another.0325

In other words, when you get what looks like a completely homogeneous thing where you can't see the individual particles of the solute to the solvent, it just looks like one thing.0327

The solvent does not have to be a liquid, but for us, we don't have to worry about that, our solvent is water.0337

OK. So let's see what we've got here. OK.0344

Sugar and salt, they both dissolve in water but do they dissolve the same way?0350

Sugar crystals, salt crystals, when you're a kid, it looks like they behave the same way.0355

It isn't until you actually get to chemistry that you discover that the dissolving process is actually completely different and what is going on inside the solution, the chemistry is entirely different.0359

Let's write that out.0372

Sugar and salt both dissolve the same way, actually, they don't dissolve the same way, they dissolve.0375

Question is "Do they dissolve the same way?" Sorry about that.0394

Both dissolve but do they do so the same way, and the answer is no, they do not.0397

Now, covalent compounds...0415

Covalent compounds are basically compounds that are made of non-metal non-metal bonds sharing of electrons.0420

Covalent compounds...they dissolve.0429

When they do so, we say that they dissolve. The example of that is sugar.0433

Sugar is a covalent compound even though it has some hydroxy groups where some of the Hs can actually be removed.0437

It is actually is considered a covalent compound because you have carbons bonded to other carbons.0444

You have carbons bonded to oxygens. You have oxygens bonded to hydrogens.0448

These are single bonds, single and double bonds.0453

When these dissolve, they just dissolve.0457

Now, salts, or ionic compounds...I'll write them as ionic compounds and of course the word "salt" is a generic term for any ionic compound.0459

I'll put salt in parenthesis.0470

Now, when salts dissolve... When these dissolve, because not all salts dissolve...Do you remember when you were doing solubility product?0473

If you take sodium chloride which is normal table salt, put it in the water, yes it'll dissolve up to a certain point.0483

If you put silver chloride into water, it'll just sink to the bottom.0489

Remember precipitation? Precipitation is salts that don't dissolve in water.0493

Now, salts when they do dissolve, they dissociate.0497

This is very, very important...they dissociate.0505

In other words, dissociation means they separate into individual ions, into individual free ions.0509

This is very, very important.0522

When a covalent can put it in there and you are not creating an electrically conductive solution.0524

But, when salt dissolves, like sodium chloride, it breaks up into Na+ and Cl- ions floating around.0532

Well, now, this solution actually will conduct electricity because you have positive and negative charges floating around.0539

The chemistry, the behaviour of the solution, is entirely different even though they actually look the same.0545

That is what's important. OK.0550

Let's just take a table sugar. Let's just sort of see what this looks like.0552

C12H22O11 - this is sucrose as a solid.0557

When we dissolve it in the water, what we end up with is C12H22O11 aqueous.0565

This aq tells us that it is dissolved.0573

This is solid, drop it in water, it is dissolved. OK.0576

That is what this aq means.0580

Let me go ahead and write that.0582

aq means dissolved. It means that is surrounded.0591

Each individual molecule of sucrose has separated from the crystal and is now surrounded by a bunch of water molecules which is why you can't see the individual crystals of the water anymore.0594

It is now a sugar solution, not a sugar crystal.0604

OK. Notice. One molecule of sugar produces one molecule of aqueous sugar.0608

1mol of the sugar crystals will produce 1mol of free individual particles.0616

These particles right here... this aqueous. That means these individual sugar molecules are floating around freely as molecules.0622

Nothing has come apart. The carbon hydrogen oxygen bonds have not broken.0630

It's just a whole molecule just floating around freely whereas here, each molecule is arranged in a crystal.0634

Now, let's go ahead and take something like magnesium chloride, a salt.0642

Magnesium chloride, an ionic compound...This is a solid.0647

When I drop this in water, what happens is it dissociates.0650

It completely comes apart into its free ions.0655

It separates into a Mg2+ ion floating around and you have two chloride ions floating around so what happens is, one unit of these...0658

We don't speak about ionic compounds as molecules because this is not really a covalent bond.0668

This is a positive charge and a negative charge that are stuck together.0674

It's a very strong bond but it's not covalent so we don’t talk about it as a molecule.0677

You can say, you can call it a unit.0682

I mean, it's not going to be the end of the world if you call it a molecule but just to let you know.0684

So 1 unit of magnesium chloride produces three particles: one magnesium ion particle and two chloride ion particles.0688

This is very, very important as you'll see in a minute.0695

Let's go ahead and put aq and aq.0701

In general, when you have ions on the right side of the arrow on an equation, the presumption is that they are aqueous, that they're dissolved,0705

unless you are specifically speaking about a gaseous phase, but we're not.0712

Everything is aqueous chemistry for us so we don't need to put the aq but I'll put them here however, in the future, I will not.0715

OK. So notice.0723

One unit of MgCl2, of the magnesium chloride, it produces three free particles.0726

That's it. That's what's going on here. Three free particles floating around in a solution.0741

Floating around in a solution...0747

Covalent compounds dissolve salts. When they do dissolve they're actually dissociating.0753

OK. Now let's talk about the notion of concentration.0760

If I take 1g of salt and I drop it into 100mL of water versus if I take 20g of salt and I drop it into 100mL of water, clearly, there is going to be a hell of a lot more salt.0765

The concentration of the solution is going to be larger in the second one. There's more salt in it.0779

The volume of the solution is the same. It still stays 100mL but I have 1g and 120g in the other.0785

I need a numerical method for differentiating between the two.0791

We call that concentration.0796

Concentration, there's a whole bunch of ways to express concentration. We're going to be concerned with two of them: molarity and percent by mass.0798

Those are the ones that I'm actually going to introduce and do examples with.0806

However, for the most part, we're really only going to be concerned with molarity - moles of solute per liters of total solution.0808

OK. So expressing concentration... Let me go ahead and put a little line here.0816

Expressing concentration... OK.0829

The first way and the primary way is something called molarity and it is expressed with a capital M.0834

Molarity is defined as the moles of solute divided by the liters of solution.0840

This is the final volume0855

Now, you remember I said that the solute doesn't have to be a solid.0857

If I take a liquid solute like liquid glucose and I drop it into liquid water, well, if I take 10mL of the liquid glucose and put it into 100mL of liquid water, now the total volume of my solution is 110mL.0860

It's not 100mL. So molecules take up volume.0875

This is liters of total solution. This is just moles of solute the things that you add.0880

It's very, very important to keep this thing straight.0885

OK. So let's do an example with molarity.0887

Again, it's all about the examples, all about working with these things using your intuition, everything that you already know from previous classes.0893

We have 7mL of lactic acid dissolved in 130mL of H2O.0902

What is the molarity of the lactic acid?0923

What is the molarity of this lactic acid solution?0928

OK. Well we'll try to write our definition.0941

The molarity is going to equal the moles of lactic acid divided by the liters of solution.0944

That is what we need.0954

We need this number, the moles of lactic acid.0956

We need the liters of solution and then we’re going to do the division and that will give us our concentration in mol/L.0958

That's the unit- mol/L.0965

Let me write that here: moles per liter and it is symbolized with a capital M.0967

I actually prefer to see my entire unit, this M thing is always...hasn't confused me but I like to work with my entire unit. I don't like anything to be hidden but that’s just a personal preference.0975

OK. So, before I do that, this is biochemistry and you know our examples.0986

We're going to try our best to use as many biological molecules as possible.0990

As we do that, I'm going to just draw out the structures of these things just so you get accustomed to seeing them and that's how we develop a sense of familiarity with these biomolecules and they are going to get larger and larger and larger.0996

So, lactic acid looks like this.1009

I'm going to do a straight carbon structure.1012

H and CH3 and we have an OH there...1020

We have three carbons: We have a carbonyl group, this is a carboxylic acid group, this is an alpha-hydroxy acid, actually, and it's an alpha-hydroxy acid.1026

Remember, this carbonyl carbon, this is the carbonyl carbon right here, the one that is attached to the double bond.1034

Let me do this in red.1039

That is the carbonyl carbon. This is called the alpha carbon and this is called the hydroxy group.1041

This is called an alpha hydroxy acid, an alpha hydroxy carboxylic acid - three carbons long.1047

This is lactic acid.1053

This is what develops in your muscles when you start to get sore, when you exercise really, really fast and the body starts to metabolize under anaerobic conditions without oxygen.1055

The by-product is actually lactic acid.1070

That's what you feel when your muscles start to get really, really sore when your exercising really, really fast.1073

OK. Now let me see what were we doing?1079

We want the molarity of this lactic acid solution.1082

OK. I'm going to keep it in red.1084

Let's do moles of... So, we need the moles of lactic acid.1087

Well, we have the milliliter of lactic acid. That is what they give us.1096

We want the moles of lactic acid.1102

How can I go from milliliter to moles?1105

Well, I know that I can go from grams of lactic acid to moles via the molar mass and I can go from milliliters to grams via the density.1107

That's my solution path. From milliliters, I'm going to go to grams and then from grams, I'm going to go to moles.1121

This is density and this is molar mass.1128

OK. Now, I look up the molar mass for lactic acid. I look up the density for lactic acid.1131

If they don't give it to me in the problem, and there is no guarantee that you're going to be given it to the problem, part of the idea is to use your resources whether they be computer resources or book resources to find the things that you need.1138

It is really, really important to be able to do that.1150

There are tables and it's very important that you become adept at utilizing your resources because there is no guarantee in real life.1152

You are just going to be presented with the problem. You have to look up these things.1162

When we look things up, the density of lactic acid is 1.209g/mL and of course I hope you will confirm this for me because I could have read it wrong, myself.1167

And, the molar mass of lactic acid is 90.08g/mol1181

We have 7mL of lactic acid times 1.209g/mL times 1mol happens to be 90.08g.1193

Now, of course, gram cancels gram, milliliter cancels milliliter and what we're left with is 0.0939mol of lactic acid.1211

You know what, I'm going to write this up. I'm just going to note moles of lactic, I'm just going to put lactic.1226

OK. Now we have the number of moles. We have the numerator, we have that.1232

Now we need the liters of solution.1238

Liters of solution...1241

OK. Well, this one is really easy.1244

We had 7mL of lactic acid.1246

We have 130mL of H2O. Both of them are liquid.1251

Remember what we said: liquid solute, liquid solvent, just add the liquids.1256

The total volume of the solution is going to be 137mL which is equivalent to 0.137L.1259

Because again, liters, moles per liter, that's the definition.1271

OK. Now, let's just go ahead and solve the problem.1275

I'll do it on the next page here.1278

So, molarity... The concentration is equal to 0.0939mol of lactic acid divided by 0.137L, 137mL, what you get is 0.686mol/L or 0.686M, molarity. There you go.1281

Either one is absolutely fine.1312

That's our concentration in moles per liter.1314

In this particular situation, in one liter of solution, you have 0.686mol. OK.1316

Now, let's introduce another expression, another way of expressing concentration.1324

This is presented by mass. This is also very, very popular.1329

You'll see this a lot on bottles at the grocery store and stuff.1331

They're expressed in terms of mass.1334

Let me go back to black here.1338

Another way to express concentration is percent by mass. OK.1344

Now, percent by mass, you'll see it this way, %m/m. That's what this symbol is for- percent by mass.1368

The definition is this: It's the mass of the solute.1380

Like any percent, it's always the part over the whole times a hundred. It's a fraction times a hundred.1387

That's what a percent is. A percent is just a fraction that has turned into a number that's a little easier to handle.1392

That's the only reason a percent exists.1398

You actually don't really need that whole multiplying by a hundred.1400

The decimal is just fine. But I guess some people just prefer numbers that are not pure decimals. OK.1404

Mass of solute over total mass of solution...1410

In other words, if I had a solution once I've made it, let's say a sugar solution that weighs 100g and of that 100g, if 5g of sugar are floating around in there, 5g of some solid sugar, what I have is 5 over 100 that is 5% sugar solution.1419

That is what this is. All percentages are just part over the whole, the part over the whole.1436

OK. Let's do an example.1441

This is going to be example 2.1446

What is the percent by mass, the %m/m of the lactic acid solution in the previous example?1451

OK. Well, we need of course the...let's do this in red again.1479

We need the mass of the solute. We need the total mass of the solution. OK.1485

Well, the mass of the lactic acid... so the mass of the lactic acid, that's just going to be the 7mL times its density.1490

We don't want to go all the way to moles so we're going to stop with grams times 1.209g/mL and when we do that we get 8.463g1501

That is the mass of the lactic acid. We have our numerator.1517

Now, total mass...1522

Well, total mass...What is our total mass?1524

Our total mass is the 8.463g of the lactic acid plus the mass of the water.1532

Well, water was 130mL. They gave it to us in volume.1542

Well, the density of water, normally, we just take it as 1g/mL so 130mL of water weighs 130g.1545

So, we have a total of 138.463 and I sure hope that you're confirming my Mathematics. I'm notorious for arithmetic mistakes.1555

OK. Well there we go. We're done.1560

So, the percent by mass of this solution is equal to 8.463g divided by...1574

Oh look at these crazy lines, can't have that. It tends to happen down at the bottom of the page so I think what I am going to do is I'm going to go ahead and move on to the next page because I don't want these crazy lines all over the place.1586

Let's try this again. So, percent by mass is equal to 8.463g of lactic acid divided by 138.463g of solution, and of course whenever we're dealing with the percent, the percent doesn’t have a unit.1598

Gram needs to cancel gram. OK. That's the whole idea...times 100 and when I do the Mathematics, I get 6.11%.1624

Our lactic acid solution was 0.686mol/L. It's 6.11%m/m.1638

That means if I had 100g of this lactic acid solution, 6.11% by mass is made up of lactic acid.1646

That's what this means. So, two ways of expressing concentration, but again, the one that we're going to be concerned with, most of the time, is going to be molarity- moles per liter.1655

And you already know that from chemistry. Most of the time, concentration, molarity is what we use. OK.1664

Well, let’s see what we've got.1673

I'm just going to list a couple of the other ways that concentration is expressed but we’re not going to be doing any samples with them because they're not going to be important for our purposes but I'd like you to know them. Ok1675

I'll go back to black here. OK.1685

Some other ways of expressing concentration had at some point or other, you certainly have heard of these or you will be hearing about them some other time in your career.1693

One of the ways is something called molality, and molality comes up when we talk about colligative properties of boiling point elevation, freezing point depression.1712

We're not going to be concerned with molality.1721

There's something called mole fraction, very, very important and again, a fraction is always the same thing. It's a part of the whole.1724

We did percent by mass. There is also something called percent by volume so it's usually designated as %v/v, I'll just go ahead and write the words out - percent by volume.1732

If I have a 100mL of solution, how many milliliters of that, what volume of that is actually the solute?1748

Percent by mass and percent by volume are two different numbers- they are not the same thing.1755

They might happen to be that same thing coincidentally but they're not the same thing.1760

And, there are other ways, I'm sure.1765

OK. Let's do another example here.1770

See if we can combine our concentration things.1777

A biochemist finds a bottle that a colleague has left at his bench that reads L-Malic 8.66%m/m.1784


What is this solution's molarity?1838


A biochemist goes to his colleague's bench and he finds this bottle that says L-Malic acid 8.66%m/m.1849

He knows that it is a Malic acid solution 8.66% by mass.1855

He wants to know what the molarity is, what is this solution’s molarity. OK.1859

In this particular case, we are going to be going from one expression of concentration to another. OK.1864

Malic acid...1869

I told you that we're going to be dealing with biological molecules so let me go ahead and draw the structure of malic acids so you see this one.1872

OK. Let's see. Shall I do it...That's fine, I'll just do it over here. OK.1880

So C,C,C,C, we have hydroxy there and then we have a wedge here.1886

Shall I put...that's fine...I'll go ahead and put the Hs in and then of course we have this.1896

So, this is malic acid. It has four carbons. It is a dicarboxylic acid. There is a carboxylic acid on one end.1902

There is a carboxylic acid on the other end and, it is also an alpha hydroxy acid because on the alpha carbon to one of the ends, there is a hydroxy and this wedge just means that it's actually coming out at us.1910

Biological molecules have a handedness, remember chirality from organic chemistry?1923

There is an L-Malic acid and there is a D-Malic acid.1928

In this particular case, it's L-Malic acid, so if we write it like this, it's actually coming out at us.1933

You will also see just will also see it this way- the regular line structure instead of this.1940

It's up to you how you want to draw, whatever is most comfortable for you.1949

I tend to draw these, I don’t really care for this very much, again, because I like to see my carbons, I like to see what I'm working with.1953

Even after all these years, I still just feel most comfortable doing it this way but of course in the books, you know you're going to see it like this, so you certainly need to be able to recognize it.1960

That is the line structure for malic acid.1974

Malic acid is what gives green apples their tartness.1977

A very, very important biological molecule as you'll find out later in the course when we discuss the citric acid cycle. OK.1981

8.66% by mass means the following.1988

They gave us the number 8.66% - that means this.1997

It means the mass of the malic acid, I'll just call it malic, over the mass of solution times 100 is equal to 8.66.2001

They gave us this. This is the definition, so, I want to start with this.2019

I'm going to work my way back.2023

OK. Now let's ask ourselves what are these that we want and let's do this in red.2027

We want molarity.2030

It's really, really important that you do know what are these that you want.2032

I want molarity, so here', molarity it means I want the moles of malic acid over the total liters of solution.2037

I need those two numbers. I need this and I need this.2043

How can I get that based on the information that I have? OK. Let's take a look at this.2057

The first thing I'm going to do is, I need the moles of malic, liters of solution.2063

The one that's quickest here...what I'm just going to do is I'm going to just...Well, let me write out what the biochemist did.2068

The biochemist measures the volume of the solution.2075

In other words, he just takes it and pours in into a graduated cylinder to see what the volume is in order to get that number.2084

measures the volume of solution to be 17.5mL.2093

In that bottle, he has 17.5mL.2104

He has the volume, he has the denominator, half our problem is done.2108

Now, we just need to find the moles of malic acid. OK.2111

That's the second part.2114

How do we find the moles of malic acid?2116

Well, I need the moles of malic acid and I know that I can get the moles of malic acid from the grams of malic acid, from the mass, from the grams of the malic acid.2119

And my conversion factor is the molar mass. Well molar mass is easy, I just looked it up. OK.2134

So the biochemist measures the mass of the solution. OK.2141

The biochemist measures the mass of the solution.2152

He puts it on the scale and he actually measures it or actually, he takes a glass vile.2156

He takes its mass. He empties the contents into that vile and then he takes that mass.2162

He subtracts the mass of the vile that he knows the mass of and he has the mass of the solution.2167

The biochemist measures the mass of the solution to be 17.75g. OK.2173

Now he has the mass of the solution.2191

Well, we need the mass of the malic acid.2195

Well, we just said earlier, the mass of malic acid divided by the mass of the solution, the total mass which is now 17.75g x 100 = 8.66.2197

We have all these numbers so now we have this equation.2216

We just do a little bit of algebra to find the mass of malic acid and that turns out to be....2218

Actually, let me do it underneath here. Sorry about that.2220

After a little bit of rearranging and a little bit of algebra, the mass of malic acid happens to be 1.537g.2234

There you go. I have 1.537g, that's my mass of malic acid.2246

I have my liters of solution - 17.5mL.2250

Now, I just have to make sure the units are appropriate.2253

Oh, I'm sorry, moles of malic acid.2258

This is my mass of malic acid. I still have to do the conversion to moles.2262

Let's do that.2265

I've got 1.537g of malic times 1mol and again, I'll look it up if the problem doesn't give it to me - 134.09g.2268

This is just basic stoichiometry, then I get 0.0115mol of malic acid.2283

OK. We are done.2293

The molarity equals 0.0115mol of malic acid divided by, and we said we had 17.5mL, right? OK. 17.75mL, the unit has to be liters so move the decimal over 3 times, we get 0.0175L2295

And when we do that, we get 0.066M or 0.66mol/L, my preferred expression for that unit.2329

That's it.2343

We were given a concentration in one expression, percent by mass, we wanted molarity, we wrote down the definition of molarity and we just took a look to see what we needed.2345

We needed moles of solute, we needed liters of solution.2356

Well, the liters of solution is easy, you just measure the volume so that it gives you that.2360

We use the information of percent by mass to recover the mass of the solute, the malic acid, and then from the mass, we use molar mass to get to moles.2363

These are the kinds of things that you want to do. Write it all down.2372

See some sort of a solution path.2376

There is only a handful of definitions. Molarity is moles per liter. Percent by mass is mass of the solute divided by total mass times 100.2379

Everything should come together really, really nicely.2388

OK. Thank you for joining us for our first lesson of biochemistry. We look forward to seeing you again. Take care.2392