The percentage composition of a molecule is just the mass percentage of each element in the molecule. To get the mass percentage, simply divide the total mass of each element in the molecule by the molecular weight of the molecule and multiply by 100%

% element = (mass element/molecular weight) *100%

For example, we can work out the mass percentage of each atom in the molecule CaCO₃, calcium carbonate. This molecule has 1 calcium atom, 1 carbon and 3 oxygen. The molecular weight is thus

Mole weight CaCO₃ = mole weight Ca + mole weight C + 3*(mole weight O)

Mole weight CaCO₃ = 40.08 g/mole + 12.01 g/mole + 3*(16.00 g/mole)

Mole weight CaCO₃ = 100.09 g/mole

The mass percentage of each is now easy to figure out:

% Ca = Mass Ca/100.09 g/mole *100% = 40.08/100.09 *100% = 40.04%

% C = Mass C/100.09 g/mole *100% = 12.01/100.09 *100% = 12.00%

% O = 3*Mass O/100.09 g/mole *100% = 3*16.00/100.09 *100% = 47.96%

Example: What is the mass percentage of potassium in potassium dichromate, K₂Cr₂O₇

Solution First, work out the molecular weight of potassium dichromate

mole weight = 2*MW_K + 2*MW_Cr = 7*MW_O

mole weight = 2*39.10 g/mole + 2*52.00 g/mole + 7*16.00 g/mole

mole weight = 294.2 g/mole

%K = 2*(mass K)/294.2 g/mole * 100%

%K = 2*(39.10 g/mole)/294.2 g/mole * 100%

%K = 26.58%

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