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.
|Write the masses||100.0 g – 51.2 g = 48.8 g||51.2g|
|Write the Mr values||120||18|
|Divide masses by Mr to find the moles||48.8 ÷ 120 = 0.406||51.2 ÷ 18 = 2.85|
|Divide by the smallest number of moles||0.406 ÷ 0.406 = 1||2.85 ÷ 0.406 = 7|
|Write the formula||MgSO4 . 7H2O|