1. Notes Ch. 10.3a Calculating Percent Composition
Chemistry

2. Percent Composition of a Compound
The percent composition is the relative amounts of the elements in a compound. It is also the percent by mass of each element in the compound.
The percent composition of a compound is made of a percent value for each different element in the compound.
Example: K2CrO4 is K = 40.3%, Cr = 26.8%, and O = 32.9% These percents must total 100%.
The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%.

3. % mass of element = mass of element/ mass of compound x 100.

4. Percent Composition from Chemical Formula
You can calculate the percent composition of a compound if you know its chemical formula. The subscripts in the formula of the compound are used to calculate the mass of each element in a mole of that compound. The sum of these masses is the molar mass
% mass = mass of element in 1 mol compound/ molar mass of compound x 100%.

5. Percent Composition as a Conversion Factor
You can use percent composition to calculate the number of grams of any element in a specific mass of a compound.
To do this multiply the mass of the compound by a conversion factor based on the percent composition.
Example:
Suppose you want to know how much carbon and hydrogen are contained in 82.0 g of propane. Percent composition says that 81.8% carbon and 18% hydrogen. That means in a g sample of propane you have 81.1 g C and 18 g H. So to calculate how much carbon is in 82.0 g C3H8 it would be: 82.0 g x 81.8 g C/100 g = g C.