Calculate the mass fraction of hydrogen in the CH4 and CaH2 compounds.

Given:
CH4
CaH2

To find:
ω (H in CH4) -?
ω (H in CaH2) -?

Solution:
1) Using the periodic system of D.I.Mendeleev, calculate the relative molecular weights of CH4 and CaH2:
Mr (CH4) = Ar (C) + Ar (H) * 4 = 12 + 1 * 4 = 16;
Mr (CaH2) = Ar (Ca) + Ar (H) * 2 = 40 + 1 * 2 = 42;
2) Calculate the mass fraction of H in CH4:
ω (H in CH4) = N * Ar (H) * 100% / Mr (CH4) = 4 * 1 * 100% / 16 = 25%;
3) Calculate the mass fraction of H in CaH2:
ω (H in CaH2) = N * Ar (H) * 100% / Mr (CaH2) = 2 * 1 * 100% / 42 = 4.76%.

Answer: Mass fraction of H in CH4 is 25%; H in CaH2 – 4.76%.