The element forms a gaseous compound of 12.5% H with hydrogen. the highest salt-like oxide is RO2

The element forms a gaseous compound of 12.5% H with hydrogen. the highest salt-like oxide is RO2, name this element.

Given:
RO2 – higher oxide
ω (H in hydrogen compound) = 12.5%

To find:
R -?

Decision:
1) Since RO2, the highest valence of the element will be equal to IV, therefore the element is located in group 4 of the periodic system of D.I.Mendeleev, which means:
H4R is its hydrogen compound;
2) Mr (H4R) = N (H in H4R) * Ar (H) * 100% / ω (H in H4R) = 4 * 1 * 100% / 12.5% = 32;
3) Mr (H4R) = Ar (H) * N (H) + Ar (R) * N (R);
1 * 4 + Ar (R) * 1 = 32;
Ar (R) = 28 – silicon (Si).

Answer: The unknown element is silicon (Si).