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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