What is the mass fraction of sodium in Na2SO4, oxygen in H2SiO3?

Given:
Na2SO4
H2SiO3

To find:
ω (Na in Na2SO4) -?
ω (O in H2SiO3) -?

Decision:
1) Mr (Na2SO4) = Ar (Na) * N (Na) + Ar (S) * N (S) + Ar (O) * N (O) = 23 * 2 + 32 * 1 + 16 * 4 = 142 ;
2) ω (Na in Na2SO4) = N (Na in Na2SO4) * Ar (Na) * 100% / Mr (Na2SO4) = 2 * 23 * 100% / 142 = 32.4%;
3) Mr (H2SiO3) = Ar (H) * N (H) + Ar (Si) * N (Si) + Ar (O) * N (O) = 1 * 2 + 28 * 1 + 16 * 3 = 78 ;
4) ω (O in H2SiO3) = N (O in H2SiO3) * Ar (O) * 100% / Mr (H2SiO3) = 3 * 16 * 100% / 78 = 61.5%.

Answer: Mass fraction of Na in Na2SO4 is 32.4%; O in H2SiO3 – 61.5%.