# How would you find molar mass?

Nov 9, 2015

Add up the atomic mass of all the atoms in the molecule

#### Explanation:

Find a periodic table and look at the atomic mass of each atom in the molecule.
I prefer to use tables with whole number atomic masses, but that is just personal preference.

Then simply find out how many of each atom is in the molecule and add the respective atomic masses to find the molecular or molar mass.

For example water ${H}_{2} O$ has 2 Hydrogen atoms (mass $1$) and 1 oxygen atom (mass $16$) so:
$\left(2 \times 1\right) + \left(1 \times 16\right) = 18$
Water has a molar mass of 18

Sulfuric acid ${H}_{\text{2}} S {O}_{4}$ has 2 Hydrogen atoms (mass $1$), 1 sulfur atom (mass $32$) and 4 oxygen atoms (mass $16$) so:
$\left(2 \times 1\right) + \left(1 \times 32\right) + \left(4 \times 16\right) = 98$
Sulfuric acid has a molar mass of 98.

The molar masses will be slightly different if you are using a table with more accurate decimal values for atomic mass. Ask your teacher or tutor if you are not sure which one to use.

Another way to find molar mass if amount and mass of a substance are known is by using the formula

$M = \frac{m}{n}$

Where $M$ is the molar mass in grams per mole ($g m o {l}^{-} 1$), $m$ is the mass of the sample in grams ($g$), and $n$ is the amount in moles of the sample ($m o l$).

For example if you had 7 moles of sulfuric acid and it had a mass of 686 grams:

$M \left({H}_{2} S {O}_{4}\right) = \frac{m}{n} = \text{686g"/"7mol} = 98 g m o {l}^{-} 1$

The equation can also be rearranged to give some other useful equations:

$n = \frac{m}{M}$

and

$m = M n$