73.5 grams C, 4.20 grams H, and 72.3 grams Cl. What is the percent composition of each element in this compound?

To find percent composition do "parts over the whole" - - - > each individual mass over the total mass times 100 (since it is a percentage).

total mass ("the whole") = 73.5 + 42.0 + 72.3 grams = 150 grams

1. Carbon % = 73.5 grams / 150 grams x 100 = 49.0%

2. Hydrogen % = 4.20 grams / 150 grams x 100 = 2.80%

3. Chlorine % = 72.3 grams / 150 grams x 100 = 48.2%