Percent Composition of Molecules

When you find a percent composition of molecules, you calculate the molar mass of an element, which is shown under each element on the periodic table, then calculate each element's percentage of that total.

formula = mass of element/ mass of compound x 100 %

Example:
What is the % composition of H2O ?

First, calculate the total molar mass of H2O

Molar mass of 2 hydrogen is 2.0 g/mole . Molar mass of oxygen is 16.0 g/mole.


So, the total molar mass of H2O is 2.0 + 16.0 = 18.0 g/mole

% H = 2.0 / 18.0 x 100% = 11.1 %
% O = 16.0 / 18.0 x 100% = 88.9 %

The results together should be added up to 100%.