Answer:
40%
Explanation:
We'll begin by obtaining the molar mass of MgO. This is illustrated below:
Molar Mass of MgO = 24 + 16 = 40g/mol
Observing the formula MgO, we have 1 atom of O in it.
The percentage composition by mass of oxygen in MgO is given by:
Mass of O/Molar Mass of MgO x 100
= 16/40 x 100 = 40%
