1.2.6 Hydrated Salts

Hydrated salt****s are ionic compounds that contain water molecules within their crystalline structure. These water molecules are called water of crystallisation. The chemical formula of a hydrated salt indicates how many water molecules are associated with each formula unit of the salt.

For example, magnesium sulfate heptahydrate has the formula MgSO₄·7H₂O, which shows that for each formula unit of magnesium sulfate, there are 7 water molecules attached.

Heating to Constant Mass

When a hydrated salt is heated, the water of crystallisation is lost, and the anhydrous (dry) form of the salt remains. By heating the hydrated salt to constant mass, you can determine the mass of water that has been removed, and subsequently calculate the molar mass ( $M_r$ ) of the hydrated salt.

How to Determine the Molar Mass of a Hydrated Salt

Weigh the hydrated salt (before heating) and record its mass.
Heat the salt to remove the water of crystallisation. Heating should be done until the mass remains constant—this ensures that all water has been removed.
Weigh the anhydrous salt after heating to determine how much water has been lost.
Calculate the mass of water lost by subtracting the mass of the anhydrous salt from the original hydrated salt mass.
Use these masses to calculate the molar mass $M_r$ of the hydrated salt.

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Example: Magnesium Sulfate Heptahydrate (MgSO₄·7H₂O) Let's say you start with 4.93 g of hydrated magnesium sulfate (MgSO₄·7H₂O) and after heating, the mass of the anhydrous magnesium sulfate is 2.41 g.

Mass of water lost:

\text{Mass of water} = 4.93 \, \text{g (hydrated salt)} - 2.41 \, \text{g (anhydrous salt)}

= 2.52 \, \text{g}

Calculate the moles of MgSO₄: The molar mass of MgSO₄ is:

M_r (\text{MgSO₄}) = 24.31 (\text{Mg}) + 32.07 (\text{S}) + 16.00 \times 4 (\text{O})

= 120.38 \, \text{g/mol}

Moles of MgSO₄:

\text{Moles of MgSO₄} = \frac{2.41 \, \text{g}}{120.38 \, \text{g/mol}} = 0.020 \, \text{mol}

Calculate the moles of water lost: The molar mass of water (H₂O) is 18.02 g/mol. Using the mass of water lost:

\text{Moles of water} = \frac{2.52 \, \text{g}}{18.02 \, \text{g/mol}} = 0.14 \, \text{mol}

Determine the formula: The ratio of moles of water to moles of MgSO₄ is:

\frac{0.14 \, \text{mol H₂O}}{0.020 \, \text{mol MgSO₄}} = 7

Therefore, the formula of the hydrated salt is MgSO₄·7H₂O.

Practical Considerations

Heating to constant mass: It is important to heat the salt multiple times and weigh it after each heating. When the mass no longer decreases, the water of crystallisation has been completely removed.
Accuracy: Ensure that no additional substances are lost during heating (e.g., decomposition of the salt itself) to maintain accuracy.

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Summary

Hydrated salts contain water molecules in their structure, which can be removed by heating. By measuring the loss of mass after heating, you can calculate the molar mass of the hydrated salt and determine the number of water molecules associated with each formula unit. This process is fundamental in stoichiometry and analytical chemistry.

Hydrated Salts Simplified Revision Notes for A-Level AQA Chemistry

1.2.6 Hydrated Salts

Heating to Constant Mass

How to Determine the Molar Mass of a Hydrated Salt

Practical Considerations

Summary

500K+ Students Use These Powerful Tools to Master Hydrated Salts For their A-Level Exams.

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