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AP®︎/College Chemistry

Course: AP®︎/College Chemistry>Unit 1

Lesson 1: Moles and molar mass

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.

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• 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.
• They are not the same thing but many people use the terms incorrectly

Atomic mass and molecular mass should be in units of u (unified atomic mass unit)

Molar mass should be in g/mol (grams per mole)
• I don't understand how Sal finds the molar mass
is there an easier way or a formula to follow to calculate it?
• 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
• why we say NaCl or KCl always why we don't say ClNa or ClK instead.
• Traditionally, the most electronegative element is written last. However, there is no harm in writing ClNa, just as long as you know that chlorine is negatively charged and sodium is positively charged.
• How come at the hydrogen is the only element not rounded off to the 2nd decimal point?
• It is probably because the atomic mass of hydrogen is so small that the extra precision makes a more significant difference when doing calculations with it.
• 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.
• 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.
• I don't really understand where the 1000 came from
• The question says it’s a 1.52 kg sample of glucose. In order to use the molar mass of glucose (~180 g/mol), the 1.52 kg needs to be converted into g first. That’s why it’s multiplied by 1000.
• 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
• 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.
• At why did Sal write 180.16, as the answer was 180.156, why didn't he write 180.15?
• 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.
• why doesn't Sal just divide 1.52 kg by the atomic mass of an single glucose atom?
• If you follow the maths of the units, it doesn’t yield mols as the answer.

1.52 kg is the total mass of the sample. And since glucose is a molecule we would want the molecular mass, not the atomic mass. The molar mass of 180.16 g/mol is the same value as the molecular mass but with different units; 180.16 amu/molecule. So if we divide the total mass by the molecular mass we get:

1.52 kg/(180.16 amu/molecule), which yields an answer with a unit of (kg*molecule)/(amu), which isn’t mols like what was asked for. You’d have to do quite a lot of conversions to get that previous answer’s unit to mols. The method Sal used in the video would give the correct answer, but with considerably less work.

Hope that helps.