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Quantitative chemistry - (CCEA) Percentage of water crystallisation in a compound

Chemists use relative atomic masses and relative formula masses to carry out mole calculations.

Part of Chemistry (Single Science)Structures, trends, chemical reactions, quantitative chemistry and analysis

Percentage of water of crystallisation in a compound

Once we have the relative mass formula of a hydrated compound, we can determine how much of this mass is water of crystallisation.

Multiply the degree of hydration by the Mr of water, and divide this by the Mr of the whole compound.

Example:

Calculate the percentage of water of crystalisation in hydrated copper (II) sulfate, CuSO4.5H2O

\({\%~water~of~crystallisation}\)

\( = \frac {degree~of~hydration \times M_r of water}{M_r~of~compound} \times 100 = \frac {5 \times 18}{250} \times 100 = 36\%\)

Determining the degree of hydration from formula - (higher tier)

A substance鈥檚 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 鈥 51.2 g = 48.8 g51.2g
Write the Mr values12018
Divide masses by Mr to find the moles48.8 梅 120 = 0.40651.2 梅 18 = 2.85
Divide by the smallest number of moles0.406 梅 0.406 = 12.85 梅 0.406 = 7
Write the formulaMgSO4 . 7H2O
Write the masses
MgSO4100.0 g 鈥 51.2 g = 48.8 g
H2O51.2g
Write the Mr values
MgSO4120
H2O18
Divide masses by Mr to find the moles
MgSO448.8 梅 120 = 0.406
H2O51.2 梅 18 = 2.85
Divide by the smallest number of moles
MgSO40.406 梅 0.406 = 1
H2O2.85 梅 0.406 = 7
Write the formula
MgSO4MgSO4 . 7H2O
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Determining the degree of hydration experimentally

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