Question

The simplest formula of a compound containing 21.9%Mg, 27.8%P, 50.3% O, by mass is.......

Answer 1

You must start by dividing each % amount by the atomic mass of the element:
Mg = 21.9/24.3 = 0.9012
P = 27.8/31 = 0.896
O = 50.3 /16 = 3.14

Divide through by smallest value ( a little rounding is permitted: )

Mg = 1
P = 1
O = 3.14/0.896 = 3.5

Multiply by 2 to remove the fraction:
Mg = 2
P = 2
O =7

Empirical formula = Mg2P2O7