Question

What is the percent by mass of oxygen in sulfur dioxide?

Answer 1

(32/64)=0.5 *100= 50%

Answer 2

EQUATION FOR % MASS:
(ELEMENT MASS IN A SAMPLE / SAMPLE MASS) x 100 = (MASS OF ELEMENT IN A MOLECULE / MASS OF MOLECULE) x 100

RAM of oxygen = 16
RAM of sulphur = 32

Sulphur dioxide has ONE sulphur atom and TWO oxygen atoms. So the mass of oxygen in one molecule:
16 x 2 = 32 Amu
Whilst the mass of SO2 is:
1 x 32 = 32 amu

Mass of molecule (relative molecular mass) = sum of atomic masses = 32 amu of oxygen + 32 amu of sulphur equals to 64 AMU (32 O + 32 S= 64 Amu)

Use the formula, we will substitute with values now:
% Mass of O = (32/64) x 100 = 50%
% Mass of S = (32/64) x 100 = 50%