# 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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## 2009 May June Paper 11 Q2

A household bleach contains sodium chlorate(I), $$NaClO$$, as its active ingredient. The concentration of $$NaClO$$ in the bleach can be determined by reacting a known amount with aqueous hydrogen peroxide, $$H _{2} O _{2}$$.
$NaClO ( aq )+ H _{2} O _{2}( aq ) \rightarrow NaCl ( aq )+ O _{2}( g )+ H _{2} O ( I )$
When $$25.0 cm ^{3}$$ of bleach is treated with an excess of aqueous $$H _{2} O _{2}, 0.0350 mol$$ of oxygen gas is given off.
What is the concentration of $$NaClO$$ in the bleach?

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