Question

If the mass percentage of water in FeSO4 X H20 is 45.3%, find the number x?

Answer 1

Take a 100 g sample.
45.3 g H2O
54.7 g FeSO4

Convert to moles.
54.7g/151.9 = 0.360
45.3/18 = 2.52

You want the FeSO4 to be 1 so we divide the FeSO4 by itself; i.e.,
0.360/0.360 = 1.00
Then to keep things equal we divide the moles H2O by the same value.
2.52/0.360 = 6.99 which rounds to 7 so the formula is
FeSO4.7H2O