AS & A Level Chemistry - 9701 Paper 5 2015 Summer Zone 2

This question concerns electrolysis of different compounds.

(a) During the electrolysis of dilute sulfuric acid using a current of 0.75A for 90 minutes, the volume of oxygen gas collected was recorded and is shown in the graph below.

(i) Give equations for the reactions that occur at each electrode in the electrolysis of sulfuric acid.
..............................................................................................................................................................
..............................................................................................................................................................
..............................................................................................................................................................
.............................................................................................................................................................. [2]

(ii) On the graph above, use a ruler to draw and label a line (hydrogen) to predict the volume of hydrogen that would be given off during the same experiment. [1]

(iii) On the graph above, use a ruler to draw and label a line (oxygen) to predict the volume of oxygen that would be produced if a current of 0.45A was used instead of the 0.75A used in the original experiment. [1]

(b) During the electrolysis of potassium butanedioate, the following reaction occurs.
$$\text{2H}_2\text{O} + \begin{array}{c}\text{CH}_2\text{COO}^-\text{K}^+ \\ \text{CH}_2\text{COO}^-\text{K}^+ \end{array} \rightarrow \text{C}_2\text{H}_4 + \text{2CO}_2 + \text{H}_2 + \text{2K}^+\text{OH}^-$$
$$\text{potassium } \text{butanedioate} $$
An experiment can be carried out to confirm the above equation. In order to do this, the amounts of hydrogen, ethene and carbon dioxide produced need to be measured.
Hydrogen is produced at one electrode, ethene and carbon dioxide are produced at the other. The carbon dioxide can be separated from the ethene by absorbing it in an alkali before the volume of ethene is measured.
(i) Using the power supply drawn below, draw a fully labelled circuit diagram and apparatus which shows how:

• the current could be measured,
• the hydrogen produced could be collected and its volume measured,
• the carbon dioxide could be removed using a named alkali,
• the volume of ethene could be measured.

In order to identify a monoprotic (monobasic) hydroxycarboxylic acid, HX, the following experiments are carried out.
25.0 cm$^3$ of an aqueous solution of HX is titrated against 0.0500 mol dm$^{-3}$ aqueous sodium carbonate. The end-point of the titration is reached when 25.0 cm$^3$ of the aqueous sodium carbonate has been added.
(a) (i) Write the equation for the complete neutralisation of HX with sodium carbonate.
........................................................................................................................................ [1]
(ii) How does the equation show that the concentration of HX is 0.100 mol dm$^{-3}$?
........................................................................................................................................
........................................................................................................................................ [1]
(b) (i) State the acid dissociation constant, $K_a$, for the above reaction in terms of H$^+$ and HX only.
\[ K_a = \]
[1]
(ii) The pH of the aqueous solution of HX is 2.43.
Use the pH and the concentration of HX to show that the $pK_a$ of the acid is 3.86. All your working must be shown. [2]

(c) In an experiment various masses of the sodium salt of the acid, NaX, are added to separate portions of 100 cm$^3$ of HX with stirring. After each addition the pH of the solution obtained is measured. The results of the experiment are recorded in the table below.
[Table_1]
\[ \begin{array}{|c|c|} \hline \text{mass of NaX added/g} & \text{pH} \\ \hline 0.00 & 2.43 \\ 0.10 & 2.81 \\ 0.20 & 3.11 \\ 0.30 & 3.19 \\ 0.40 & 3.41 \\ 0.60 & 3.59 \\ 0.80 & 3.71 \\ 1.00 & 3.81 \\ 1.20 & 3.89 \\ 1.50 & 3.99 \\ 2.00 & 4.11 \\ \hline \end{array} \]
(i) Plot a graph to show how the pH of the solution varies with the mass of NaX added. Draw the curve of best fit. [2]
(ii) Circle one anomalous point on your graph and give a reason for how this could have occurred using the experimental procedure described.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]

(d) (i) The graph shows that a pH of 3.86 is obtained when 1.12 g of NaX is added to 100 cm$^3$ of HX. Remember that $pK_a$ of HX is also 3.86. Use this information to calculate the relative molecular mass, $M_r$, of HX. Show your working.
[A_r: H, 1.0; C, 12.0; O, 16.0; Na, 23.0] [3]
(ii) The calculated $M_r$ is subject to small experimental error. Suggest a structure for the organic hydroxycarboxylic acid, HX, that best fits your $M_r$ data.
If you have not calculated a value for the $M_r$, use the value of 104. This is not the correct value. [1]

(e) Another method for determining the concentration of the acid HX could be to evaporate a sample of the solution and weigh the solid that remains.
Suggest two reasons why this might not be a very good method of finding the mass of solid HX in a sample of the solution.
........................................................................................................................................
........................................................................................................................................
........................................................................................................................................ [2]