Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

A student is given a sample of an organic acid, V, and is told to work out its relative molecular mass and propose its molecular formula.
(a)[1]

Calculate the mass of V used in the experiment.

(b)[1]

What is the colour of the solution in the conical flask before the acid is added and at the end-point?

(c)[4]

Use the diagrams to fill in the titration table and determine the mean volume of $0.100\,\text{mol dm}^{-3}$ hydrochloric acid used.

(d)[1]

Calculate the number of moles of hydrochloric acid in the average volume of $0.100\,\text{mol dm}^{-3}$ hydrochloric acid from (c).

(e)[1]

Using the equation $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ and your answer from (d), determine the number of moles of sodium hydroxide in $25.0\,\text{cm}^3$ of W.

(f)[1]

Using your answer from (e), calculate the number of moles of sodium hydroxide in $250\,\text{cm}^3$ of W.

(g)[1]

Calculate the number of moles of sodium hydroxide in $50\,\text{cm}^3$ of $1.00\,\text{mol dm}^{-3}$ sodium hydroxide.

(h)[1]

By subtracting your answer in (f) from your answer in (g), calculate how many moles of sodium hydroxide reacted with the original sample of the organic acid, V.

(i)[1]

One mole of V reacts with two moles of sodium hydroxide. Deduce the number of moles of V in the sample.

(j)[1]

Using your answers from (a) and (i), calculate the relative molecular mass of the acid V.

(k)[2]

The acid V contains two carboxylic acid groups and has the molecular formula $\text{HO}_2\text{C}_x\text{H}_y\text{CO}_2\text{H}$. Deduce the values of $x$ and $y$ in the molecular formula.

(l)[1]

Give the structure of the ester produced when V reacts with two molecules of ethanol under suitable conditions.

Worked solution & mark scheme

This 16-mark question has a full step-by-step worked solution and mark scheme. One marking point: 1.46 g is the correct value

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