Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

A $6.30\,\text{g}$ portion of hydrated ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$, was dissolved in water and the resulting solution was made up to $250\,\text{cm}^3$. A $25.0\,\text{cm}^3$ aliquot of this solution was acidified and titrated with $0.100\,\text{mol dm}^{-3}$ potassium manganate(VII) solution. $20.0\,\text{cm}^3$ of this potassium manganate(VII) solution was needed to react completely with the ethanedioate ions, $\text{C}_2\text{O}_4^{2-}$, in the sample.
(a(i))[1]

The $\text{MnO}_4^-$ ions in the potassium manganate(VII) oxidise the ethanedioate ions. Explain, using electron transfer, what the word oxidise means in the statement above.

(a(ii))[3]

Complete the ionic equation for the reaction of the manganate(VII) ions with the ethanedioate ions, balancing it fully. $2\text{MnO}_4^-\text{(aq)} + 5\text{C}_2\text{O}_4^{2-}\text{(aq)} + \ldots\ldots\text{H}^+\text{(aq)} \rightarrow \ldots\text{(aq)} + 10\text{CO}_2\text{(aq)} + \ldots\ldots\text{H}_2\text{O(l)}$

(b(i))[1]

Calculate how many moles of manganate(VII) are used in the titration.

(b(ii))[1]

Use the equation in a(ii) together with your answer to b(i) to calculate how many moles of $\text{C}_2\text{O}_4^{2-}$ are present in the $25.0\,\text{cm}^3$ sample of solution used.

(b(iii))[1]

Calculate how many moles of $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$ are in $6.30\,\text{g}$ of the compound.

(b(iv))[1]

Calculate the relative formula mass of $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$.

(b(v))[1]

The relative formula mass of anhydrous ethanedioic acid, $\text{H}_2\text{C}_2\text{O}_4$, is $90$. Calculate the value of $x$ in $\text{H}_2\text{C}_2\text{O}_4\cdot x\text{H}_2\text{O}$.

Worked solution & mark scheme

This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: $\mathrm{C}_2\mathrm{O}_4^{2-}$ ions lose electrons (are oxidised)

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