# How do you calculate the moles of a substance?

Oct 15, 2014

In order to calculate the moles of a substance, you need to know the mass of the substance and its molar mass. Molar mass is the atomic weight in grams/mol.

Example:
How many moles of copper(II) sulfate, ${\text{CuSO}}_{4}$, are in ${\text{250.0g CuSO}}_{4}$?

Molar Mass of ${\text{CuSO}}_{4}$
Subscript x molar mass =
1 x 63.456g/mol Cu = 63.456g/mol Cu
1 x 32.065g/mol S = 32.065g/mol S
4 x 15.999g/mol O = 63.996g/mol O
Total: 159.517g/mol

1mol ${\text{CuSO}}_{4}$ = 159.517g ${\text{CuSO}}_{4}$, which gives us two conversion factors:

$\text{1mol CuSO"_4/"159.517g CuSO"_4}$ and ${\text{159.517g CuSO"_4/"1mol CuSO}}_{4}$

Now multiply the known mass x the conversion factor with moles on top and grams on bottom. This will cancel the grams and leave the moles.

${\text{250.0g CuSO}}_{4}$ x $\text{1mol CuSO"_4/"159.517g CuSO"_4}$ = ${\text{1.567mol CuSO}}_{4}$