Percent of composition is the amount each element makes up part of a compound.

Example Problem

Given the compound ZnCl₂ 

What is the percent of each element in the compound?

First find the molar mass of ZnCl₂

Zn is 65.39 g/mol

Cl is 2 x 35.45 g/mol

Total is 136.29 g/mol for ZnCl₂

Zn percent is  65.39 g/mol / 136.29 g/mol  X 100 % = 47.98 % for Zn

Cl percent is 70.90 g/mol / 136.29 g/mol X 100 % =  52.02 % for Cl