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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