Quantitative chemistry - (CCEA)Degree of hydration from formula - Higher tier

We looked at calculating and using moles for solids using mass in unit 1.7. Here we are using moles of a solute dissolved in a solution and moles of a gas to carry out calculations.

Part ofCombined ScienceFurther chemical reactions, rates and equilibrium, calculations and organic chemistry

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
Write the masses
MgSO4100.0 g – 51.2 g = 48.8 g
H2O51.2g
Write the Mr values
MgSO4120
H2O18
Divide masses by Mr to find the moles
MgSO448.8 ÷ 120 = 0.406
H2O51.2 ÷ 18 = 2.85
Divide by the smallest number of moles
MgSO40.406 ÷ 0.406 = 1
H2O2.85 ÷ 0.406 = 7
Write the formula
MgSO4MgSO4 . 7H2O
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Mole calculations in solutions - Higher tier
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Determining the degree of hydration experimentally

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