Finding the molar mass of a compound is often the first step in more complicated chemistry
problems.
When we say “molar mass,” we mean the mass of 1 mole of that compound.
Sometimes it’s called the Gram molecular mass . I’ll also explain what is meant by
gram formula mass.
That’s a lot of terms that all sort of mean the same thing - so it’s no wonder people
get confused.
We’ll work through 5 examples.
You’ll need 1.
The formula of the compound and 2.
A periodic table, to know the gram atomic mass of each atom.
Here’s our first example: What’s the molar mass of water?
We could also say - what’s the gram molecular mass of water - because water is a molecule.
We’re trying to find what’s the mass of one mole of water molecules - in grams.
The formula of water is H2O.
To calculate the mass of one molecule of water, you would add the atomic masses of the 3 atoms
that make up one water molecule.
But we don’t want the mass of just one molecule of water.
We want the mass of one MOLE of water molecules.
BTW, we have a short glossary video about the mole, if you need a refresher for that
definition.
1 mole of H atoms = 1.0 g H (I got that from the periodic table - we usually go to 1 decimal
place for these calculations) 1 mole of O atoms = 16.0 g O (Again, that’s
from periodic table).
The molecular formula of water is H2O, which tells us there are 2 moles of H atoms and
one mole of O atoms in 1 mole of water molecules.
Let’s add that all up.
In one mole of water molecules, there are 2 mol H atoms (1.0 gH/ 1 mol H) = 2.0 g
1mol O (16.0 g O/ 1 mol O) = 16.0 g ....
18.0 g/mol = molar mass of water.
We can also call this the gram molecular mass of water.
Let’s do another example that is similar.
What’s the molar mass of glucose?
The formula for glucose is C6H12O6.
Look on the periodic table to find the gram atomic masses for all of the atoms in this
molecule.
Carbon is 12.0 g/mol, hydrogen is 1.0 g/mol, and oxygen is 16.0 g/mol.
Multiply by the number of each atom, like so.
(6)(12.0) + (12)(1.0) + (6)(16.0) = 180.0 g/mol That’s the molar mass of glucose.
Or you may hear it called the gram molecular mass.
Sometimes you’ll hear the term “gram formula mass” instead of “gram molecular mass.”
What’s the difference between these two terms?
Both are molar mass - you calculate them in the exact same way.
But “gram molecular mass” refers to compounds that are molecules, like water, and glucose,
things that are held together by covalent bonds.
“Gram formula mass” refers to ionic compounds.
Table salt, for instance - NaCl - that’s an ionic compound.
And the “representative particle” of an ionic compound is called a “formula unit”
as opposed to a “molecule.”
Hence the name “gram formula mass.”
So let’s do one of those, and you’ll see it’s really the same kind of problem.
If someone asks you for the molar mass of table salt (or they ask you for the “gram
formula mass” of table salt) - you need, once again the same two things:
1.
formula of the compound: NaCl 2.
periodic table (to look up the atomic masses).
There’s one sodium ion and one chloride ion in each formula unit of NaCl, so there’s
no multiplying involved here.
Gram atomic mass of sodium (23.0 g/mol from the periodic table) + gram atomic mass of
chlorine (35.5 g/mol, again, from the periodic table) = 58.5 g/mol.
Let’s do a couple typical applications where you would use the molar mass in chemistry
problems.
4.
{Application using molar mass} How many moles of water are in 50.0 grams of water?
Earlier, we calculated the molar mass of water - 18.0 g/mol.
We’re going to use this as a conversion factor.
If you’re a little uncomfortable with the idea of using conversion factors, please watch
our 2 videos about unit conversion and dimensional analysis - they tell you all about using conversion
factors and this style of problem solving.
[9] We start with what we know, 50.0g H2O on the left - leave a big space, and write
what we want on the right = _____moles H2O.
We multiply 50.0 g by a conversion factor to get to moles of water.
Our conversion factor will have g of H2O and moles of H2O in it.
That’s the molar mass: There’s 18.0 g in 1 mol H2O.
We’ll write it as a fraction equal to 1.
So our conversion factor is 18.0 g/1 mol.
But remember,we have to be careful to put that conversion factor in the right orientation
so the units cancel.
If we write it as 1 mol/18.0g, then grams cancel.
Multiply across the top, and multiply across the bottom.
50.0g H2O (1 mol/18.0 g) = 50.0/18.0 mol H2O.
This simplifies to 2.78 mol H2O
A combustion reaction consumes 12.6 moles of glucose C6H12O6.
How many grams of glucose is that?
Start with what you know on the left.
12.6 moles of C6H12O6.
Leave space for your conversion factor.
Now write what you want in your answer.
= _____ g C6H12O6.
We need a conversion factor that will take us from moles of glucose to grams of glucose.
We can use the molar mass of glucose to do that.
The molar mass of glucose is 180.0 g per 1mol (we calculated that in example 2).
We’ll write that as a fraction equal to 1, to use it as a conversion factor.
180.0 g/1 mol.
Be careful with the orientation of the conversion factor so units cancel:12.6 moles C6H12O6
(180.0g/1mol) = ____g C6H12O6.
moles cancel.
Multiply across and you’ll get (12.6)(180.0)/1 = 2268 g C6H12O6