If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Worked example: Calculating molar mass and number of moles

The molar mass of a substance is the mass in grams of 1 mole of the substance. As shown in this video, we can obtain a substance's molar mass by summing the molar masses of its component atoms. We can then use the calculated molar mass to convert between mass and number of moles of the substance. Created by Sal Khan.

Want to join the conversation?

  • leafers ultimate style avatar for user Banana bomb
    Is molar mass the same as molecular mass, or do they differ?
    In some websites they say it's different and some say it's same.
    (29 votes)
    Default Khan Academy avatar avatar for user
  • leafers ultimate style avatar for user Banana bomb
    I don't understand how Sal finds the molar mass
    is there an easier way or a formula to follow to calculate it?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • mr pants purple style avatar for user Ryan W
      The molar mass of any element is on the periodic table.
      For a molecule or compound, simply add up all the molar masses of the elements, taking subscripts into account.

      Eg for MgCl2 it would be equal to Mg + 2 x Cl = 24.305 + (2 x 35.45) = 95.21
      (28 votes)
  • piceratops ultimate style avatar for user Everett Garrett
    How come at the hydrogen is the only element not rounded off to the 2nd decimal point?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user asif.ejaz005
    why we say NaCl or KCl always why we don't say ClNa or ClK instead.
    (4 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Sarah
    I don't understand finding the significant figures at the end of the example. What are significant figures, and what determines how many significant figures we round our final answer to? Maybe they've already gone over it and I just don't remember.
    (8 votes)
    Default Khan Academy avatar avatar for user
    • mr pants purple style avatar for user Ryan W
      The basic idea is that your answer to a calculation shouldn’t have more significant figures than the initial quantity given has. The initial quantity was 1.52 kg, that has 3 significant figures, so the answer should be given to 3 significant figures too.
      (9 votes)
  • aqualine tree style avatar for user rbangura
    I don't really understand where the 1000 came from
    (3 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Miriam  Biros
    How would you solve something like:
    What is the mass of
    3.5 mol of FeCl2

    and

    What is the mass of
    5 x 10^-4 mol H2SO3
    (4 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user David Park
      First, you can calculate the molar mass of FeCl2 by adding the molar masses of Fe (55.845 g/mol) and 2 atoms of Cl (2 times (35.446 g/mol). This gives a molar mass of 126.737 g/mol. Since each mole is 126.737 grams, you multiply 3.5 mols by 126.737 grams, giving you 443.58 grams.
      (9 votes)
  • piceratops ultimate style avatar for user Sohail
    At why did Sal write 180.16, as the answer was 180.156, why didn't he write 180.15?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user Richard
      It's important to keep in mind significant figures are important for doing calculations in a science like chemistry. After a calculation would be able to use all the digits of the final number as our answer if we only wanted a mathematical answer. But in science we are more conservative with the digits we use because of the precision of our measuring instruments. And the rules we use to judge how many digits are permissible are significant figures, or sig figs.

      You should look into sig figs in greater detail, but in this problem Sal is adding three numbers together. For addition the sig fig rule is that we can only have as many decimal digits as do in the calculation number with the lowest number of decimal digits.

      So after multiplying Sal is performing this calculation: 72.06 + 84.156 + 96.00, which mathematically would yield 180.156. But because we only have two decimals digits for the 72.06 and 96.00 numbers, our answer is limited to two decimal digits. So we only want the 1 and 5 digits and want to discard the 6. Since the number we're discarding (6) is larger than 4 we round the digit it is next to up one number. So the 5 digit get upgraded to a 6 in the answer. So the final answer should be reported as 180.16 g/mol.

      Hope that helps.
      (7 votes)
  • aqualine sapling style avatar for user luqmanulhakim1412
    hi guys, i have a question,

    why do this calculation didnt include avogadro number like the previous video?

    thanks for helping :D
    (3 votes)
    Default Khan Academy avatar avatar for user
  • starky ultimate style avatar for user peyball2008
    Can someone please just walk me through how to convert grams to atoms because I am so lost.
    (5 votes)
    Default Khan Academy avatar avatar for user

Video transcript

- [Instructor] We are asked to calculate the number of moles in a 1.52 kilogram sample of glucose. So like always, pause this video and try to figure this out on your own and this periodic table of elements will prove useful. All right, now if we're trying to figure out the number of moles, remember, mole is really, you can view it as a quantity of something. If I said a dozen of something, you'd say oh, that's 12 of that thing. If I say a mole of something, I'm saying that's Avogadro's number of that thing. And so we have a 1.52 kilograms sample of our molecule in question, of glucose so if we can figure out the mass per mole, or another way to think about it, the molar mass of glucose, well then we just divide the mass of our sample by the mass per mole and we'll know how many moles we have. So what is the molar mass of glucose? Well to figure that out, and that's why this periodic table of elements is useful, we just have to figure out the molar mass of the constituent elements. So if we first look at carbon, carbon, we see from this periodic table of elements, has a molar mass of 12.01 grams per mole. We've talked about it in other videos, you could view this 12.01 as a relative atomic mass of a carbon atom, of as the average atomic mass of a carbon atom, or what's useful, and this is where Avogadro's Number is valuable, if you have Avogadro's Number of carbons, it is going to have a mass of 12.01 grams. So carbon has a molar mass of 12.01 grams per mole and now we can think about hydrogen in the same way. Hydrogen has a molar mass of 1.008 grams per mole, 008 grams per mole. And then last but not least, we have oxygen here. Oxygen, we can see from our periodic table of elements, it has a molar mass of 16.00 grams per mole. And so now we have all the information we need from our periodic table of elements. So the molar mass of glucose is going to be six times the molar mass of carbon plus 12 times the molar mass of hydrogen plus six times the molar mass of oxygen. So it's going to be six times 12.01 grams per mole plus 12 times 1.008 grams per mole plus every molecule of glucose has six oxygen plus six times 16.00 grams per mole. Six times 12.01 plus 12 times 1.008 plus six times 16 is equal to, and if we're thinking about significant figures here, the molar mass of hydrogen goes to the thousandths place but we only go to the hundredths for carbon and for oxygen, we're adding all of these up together so it's going to be 180. I can only go to the hundredths place for significant figures, so 180.16. So that's equal to 180.16 grams per mole. And we could say grams of glucose, C6H12O6 per mole of glucose, C6H12O6 and then we can use this 1.52 kilograms to figure out how many moles we have. So if we start off with 1.52 kilograms of glucose, so that's C6H12O6, well first we can convert it to grams 'cause here, our molar mass is given in terms of grams, so times, we're going to want kilograms in the denominator and grams in the numerator, so how many grams are there per kilograms? Well, we have 1,000 grams for every one kilogram. So when you multiply these two out, this is going to give you the number of grams we have of glucose which would be 1,520 and if you have your mass in terms of grams, you can then divide by your molar mass or you can view it as multiplying it by the moles per gram. So for every one mole of glucose, C6H12O6, we have 180.16 grams of glucose, C6H12O6, and this is going to get us, we get 1.52 times 1,000 is equal to, this is the number of grams of glucose we have, and then we're going to divide by 180.16, divide by 180.16, gives us this number, and let's see, if we see significant figures, we have three significant figures here, we have five here so we wanna round it to three significant figures, so it will be 8.44 moles of glucose. So our kilograms cancel with our kilograms and then our grams of glucose cancel with our grams of glucose and we are left with 8.44 moles of glucose, moles of C6H12O6. And we are done.