EasyChemistryHigh School (11–12)The p-Block Elements › Groups 13 and 14: Boron and Carbon FamiliesNumericalA-Level IB

Potash alum is KAl(SO₄)₂· 12H₂O with a molar mass of 474.37 g/mol…

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Potash alum is $\text{KAl}(\text{SO}_{4})_{2}\cdot 12\text{H}_{2}\text{O}$ with a molar mass of $474.37\ \text{g/mol}$ . Take the molar mass of water as $18.015\ \text{g/mol}$ and of aluminium as $26.98\ \text{g/mol}$ . Find the percentage of water of crystallisation, the percentage of aluminium and the mass of alum that holds $1.00\ \text{g}$ of aluminium.

Need a reminder? The key ideaShow

Faraday's law for aluminium: $m = \dfrac{M\,Q}{3F}$ with $Q = It$ . Energy in kilowatt hours is $\dfrac{V\,Q}{3.6 \times 10^6}$ . Thermite: $2\text{Al} + \text{Fe}_{2}\text{O}_{3} \to \text{Al}_{2}\text{O}_{3} + 2\text{Fe}$ gives out $851.5\ \text{kJ}$ for 2 mol of Al. A hydrate has its water in the molar mass.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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