What information do you need to calculate percent composition by mass?

Question

% Composition Question 

Answer 1

You need only to have the chemical formula to calculate the percent composition by mass. 

 

If you know each element in the formula, then you can calculate total molar mass of the compound.  Once you have that value, you can calculate the percent by mass of each individual element by dividing its mass by the total mass and multiplying by 100%.