How do I calculate the percent composition of the following by mass with 3 significant figures O in Na2SO4?

Answer 1

Add up the compound's molar mass: Na=2(23), S=1(32), O=4(16). Sum is 142. Oxygen's part of that total is 64 (16x4). Percent is part/total, so 64/142 = 0.4507 = 45.1%.

Answer 2

The molar mass of sodium sulfate is 142.
The percent composition is:
- oxygen: 45,1 %
- sulfur: 22,5 %
- sodium: 32,4 %