Determine the mass fractions of potassium, nitrogen and oxygen in potassium nitrate KNO3.

Given:
KNO3 – potassium nitrate

To find:
ω (K in KNO3) -?
ω (N in KNO3) -?
ω (O in KNO3) -?

Decision:
1) Mr (KNO3) = Ar (K) * N (K) + Ar (N) * N (N) + Ar (O) * N (O) = 39 * 1 + 14 * 1 + 16 * 3 = 101 ;
2) ω (K in KNO3) = N (K in KNO3) * Ar (K) * 100% / Mr (KNO3) = 1 * 39 * 100% / 101 = 38.6%;
3) ω (N in KNO3) = N (N in KNO3) * Ar (N) * 100% / Mr (KNO3) = 1 * 14 * 100% / 101 = 13.9%;
4) ω (O in KNO3) = 100% – ω (K in KNO3) – ω (N in KNO3) = 100% – 38.6% – 13.9% = 47.5%.

Answer: Mass fraction of K is 38.6%; N 13.9%; O – 47.5%.