A metal (M) forms an oxide with the formula MO. If the oxide contains 7.17 % O by mass, what is the identity of the metal?

The metal is lead, Pb.

One unit of the oxide contains one atom of O (16.00 u).

∴ Mass of oxide = 16.00 u O * (100 u MO/7.17 u O) = 223.15 u MO

Mass of M = mass of MO - mass of O = 223.15 u - 16.00 u = 207.2 u

The only element with an atomic mass of 207.2 u is lead (Pb) and the formula of the oxide is PbO.