# How do you calculate percent composition of a compound?

May 21, 2015

So you calculate the weight of the entire molecule (molecular weight/ formula weight)

The you add the weight of one of the contributing elements.

You put the small mass over the big mass and times by 100%.

e.g. ${H}_{2} O$
Molecular weight = 18
weight of H = 2
weight of O = 16

% composition H =$\left(\frac{2}{18}\right) \cdot 100$