How do you calculate the moles of a substance?

1 Answer
Meave60
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, "CuSO"_4, are in "250.0g CuSO"_4?

Molar Mass of "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 "CuSO"_4 = 159.517g "CuSO"_4, which gives us two conversion factors:

"1mol CuSO"_4/"159.517g CuSO"_4" and "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.

"250.0g CuSO"_4 x "1mol CuSO"_4/"159.517g CuSO"_4" = "1.567mol CuSO"_4

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