# Stoichiometry Calculation

## 2020 Oct Nov Paper 12 Q5

A white powder is known to be a mixture of magnesium oxide and aluminium oxide.

100 cm3 of 2 mol dm-3 NaOH (aq) is just enough to dissolve the aluminium oxide in $$x$$ grams of the mixture.

The reaction is shown.
$Al _{2} O _{3}+2 OH ^{-}+3 H _{2} O \rightarrow 2 Al ( OH )_{4}^{-}$
800 cm3 of 2 mol dm-3 HCl (aq) is just enough to dissolve all of the oxide in $$x$$ grams of the mixture.
The reactions are shown.
\begin{aligned} Al _{2} O _{3}+6 H ^{+} & \rightarrow 2 Al ^{3+}+3 H _{2} O \\ MgO +2 H ^{+} & \rightarrow Mg ^{2+}+ H _{2} O \end{aligned}
How many moles of each oxide are present in $$x$$ grams of the mixture?

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