Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

A student works out the concentration of aqueous potassium manganate(VII), $\text{KMnO}_4$, by carrying out a titration with ethanedioic acid solution, $\text{H}_2\text{C}_2\text{O}_4$. The reaction may be represented by the equation shown below. $2\text{KMnO}_4 + 5\text{H}_2\text{C}_2\text{O}_4 + 6\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 10\text{CO}_2 + 8\text{H}_2\text{O} + 2\text{K}^+$
(a)[3]

Some of the apparatus used by the student are drawn below. Name the three pieces of apparatus A, B and C.

(b(i))[1]

Suggest why sulfuric acid is introduced into apparatus C.

(b(ii))[1]

State the colour change at the end-point, from [BLANK] to [BLANK].

(b(iii))[3]

Enter the values in the results table. Complete the results table.

(b(iv))[1]

Tick (✓) the best titration results in the table. Use the ticked values to find the average volume of $\text{KMnO}_4\text{(aq)}$ added in cm$^3$.

(c(i))[2]

Calculate the number of moles of ethanedioic acid present in $25.0\ \text{cm}^3$ of the $\text{H}_2\text{C}_2\text{O}_4\text{(aq)}$. $[M_r:\ \text{H}_2\text{C}_2\text{O}_4,\ 90]$

(c(ii))[1]

The stoichiometric ratio shows that two moles of $\text{KMnO}_4$ react with five moles of $\text{H}_2\text{C}_2\text{O}_4$. Calculate the number of moles of $\text{KMnO}_4$ present in $24.55\ \text{cm}^3$ of the aqueous potassium manganate(VII).

(c(iii))[1]

Calculate the concentration, in $\text{mol dm}^{-3}$, of the $\text{KMnO}_4\text{(aq)}$.

(c(iv))[1]

Calculate the concentration, in $\text{g dm}^{-3}$, of the $\text{KMnO}_4\text{(aq)}$. Give your answer to three significant figures. $[M_r:\ \text{KMnO}_4,\ 158]$

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