Determining the degree of hydration from formula - Higher tier

A substance’s degree of hydration can be calculated from percentage composition or mass composition. The method is similar to other empirical formula questions. Work with the anhydrous salt in the first column and water in the second. There are only ever two columns.

Hydrated magnesium sulfate contains 51.2 % water of crystallisation. Determine the degree of hydration and write the formula of hydrated magnesium sulfate.

MgSO4H2O
Write the masses100.0 g – 51.2 g = 48.8 g51.2g
Write the Mr values12018
Divide masses by Mr to find the moles48.8 ÷ 120 = 0.40651.2 ÷ 18 = 2.85
Divide by the smallest number of moles0.406 ÷ 0.406 = 12.85 ÷ 0.406 = 7
Write the formulaMgSO4 . 7H2O