If the oxide contains 7.17 % O by mass, what is the identity of the metal?

Answer is: the identity of the metal is lead (Pb).

ω (O) = 7.17% : 100%.

ω (O) = 0.0717; mass percentage of oxygen in the oxide.

If we take 100 grams of compound:

1) m (O) = ω (O) · 100 g.

m (O) = 0.0717 · 100 g.

m (O) = 7.17 g; mass of oxygen in the oxide.

n (O) = m (O) : M (O).

n (O) = 7.17 g : 16 g/mol,

n (O) = 0.448 mol; amount of oxygen.

2) m (M) = 100 g - 7.17 g.

m (M) = 92.83 g; mass of metal in the oxide.

n (O) = n (M) = 0.448 mol.

M (M) = m (M) : n (M).

M (M) = 92.83 g : 0.448 mol.

M (M) = 207.20 g/mol; molar mass of the metal.