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## Chemistry library

### Course: Chemistry library>Unit 1

Lesson 2: Ions and compounds

# 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?

• 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 ,molar mass is equal to mass per mole? Like molar mass of H in gram is 1 gm/1 mole ?
• This is true! This is the case because 1 mole of a molecule is equal to 6.02214076×10^23 (avogadros constant) individual molecules. Avogadros constant is specifically chosen so 1u (or 1 dalton) is equal to 1 gram/mole. (see https://www.wikiwand.com/en/Avogadro_constant in the 3rd paragraph)
• Sal added g/mol at the end of every decimal number. Do I have to do the same when I write the equation out? Or is it optional?
• it is not exactly optional it simply means grams per mol this means it like the S.I unit
(1 vote)
• At why did Sal write 180.16, as the answer was 180.156, why didn't he write 180.15?