What is the percent composition by mass of oxygen in Ca (NO3) 2 (gram-formula mass=164 g/mol) ?

(1) 9.8% (3) 48%

(2) 29% (4) 59%

Solving for the formula mass of calcium nitrate: Atom | Atomic Mass Ca | 1 x 40.08 = 40.08 N | 2 x 14.01 = 28.02 O | 6 x 16 = 96 FM = 164.10 amu Solving for the percent: %O2 = mass of oxygen / mass of compound x 100% %O2 = 96 amu / 164.10 amu x 100% %O2 = 58.50% or 59%