Calculate the percent water in iron ii sulfate heptahydrate

According to its formula FeSO4.7H2O

we can get the percent % by the mass of H2O from this formula

%mass of H2O = (mass of water H2O / mass of the hydrate) x100

when the mass of water = molar mass x 7 = 18 x 7 = 126

and the mass of hydrate (feSO4) = molar mass = 278

So by substitution:

%mass of H2O = (126/278) x 100 = 45%