Law of Definite Proportions: Elements in a chemical compound are present in the same proportions by mass.
Percentage composition = (mass of element/total mass) X 100
This percentage composition can help us solve what compounds we are looking at in some problems. Percentage composition can also help distinguish between compounds with the same component elements but in different proportions (ex. H2O vs H2O2).
For homework, please do questions 1-4 on page 82 and questions 5-8 on page 85.
Some things to be careful of: make sure that you are carefully summing the total mass of your compound- if you don’t have the correct compound mass, all your percentage composition calculations will be off.