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(a) (i) Calculate the mass of $(NH_4)_2S_2O_8(s)$ that would be required to prepare 250$cm^3$ of a standard solution of concentration 1.00 $mol dm^{-3}$.
[A_r values: N, 14.0; H, 1.0; S, 32.1; O, 16.0]
mass of $(NH_4)_2S_2O_8(s)$ = .............................. g [1]
(ii) Describe how, after weighing the mass calculated in (i), you would prepare this standard solution for use in your experiment.
Give the name and capacity, in $cm^3$, of any apparatus used.
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(iii) Explain how the use of starch solution improves the accuracy of the experiment.
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(b) A student planned five experiments to investigate the effect of iodide concentration, $I^-$, on the rate of reaction. The table shows the volumes used in experiment 1.
Complete the table for experiments 2 to 5.
| experiment | volume of $1.00 \ mol \ dm^{-3} \ (NH_4)_2S_2O_8(aq) \, / \, cm^3$ | volume of $0.20 \ mol \ dm^{-3} \ KI(aq) \, / \, cm^3$ | volume of water / $cm^3$ | volume of $0.0050 \ mol \ dm^{-3} \ Na_2S_2O_3(aq) \, / \, cm^3$ | volume of starch solution / $cm^3$ |
|---|---|---|---|---|---|
| 1 | 25.0 | 10.0 | 0.0 | 5.0 | 1.0 |
| 2 | 5.0 | 1.0 | |||
| 3 | 5.0 | 1.0 | |||
| 4 | 5.0 | 1.0 | |||
| 5 | 5.0 | 1.0 |
(c) In a different experiment, a student mixed the following solutions and measured the time taken for the reaction.
• 10.0$cm^3$ of 1.00 $mol dm^{-3}$ $(NH_4)_2S_2O_8(aq)$
• 5.0$cm^3$ of 0.0050 $mol dm^{-3}$ $Na_2S_2O_3(aq)$
• 5.0$cm^3$ of 0.20 $mol dm^{-3}$ $KI(aq)$
• 1.0$cm^3$ of starch indicator
(i) The time taken for the blue colour to appear was 134 seconds (to the nearest second).
Calculate the rate of production of moles of $I_2$, in $mol dm^{-3} s^{-1}$.
rate of production of moles of $I_2$ = ................................................... $mol dm^{-3} s^{-1}$ [3]
(ii) What should the student have done to make sure that the results were reliable?
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(iii) The 5.0$cm^3$ of 0.0050 $mol dm^{-3}$ $Na_2S_2O_3(aq)$ was measured using a 50$cm^3$ burette which had graduations every 0.1$cm^3$.
Calculate the maximum percentage error in the measured volume of this solution.
percentage error = ................................ % [1]
(d) A second student tried to perform the same experiment but found that the reaction mixture turned blue immediately after $KI(aq)$ was added.
State what error the student had made.
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(e) The following information gives some of the hazards associated with the chemicals used in the procedure.
| Ammonium persulfate Solid is oxidising and hazardous to the environment. Contact with combustible material may cause fire. It is classified as health hazard, is harmful if swallowed and is irritating to eyes, respiratory system and skin. Solutions equal to or more concentrated than 0.2$mol dm^{-3}$ should be labelled health hazard and hazardous to the environment. Solutions equal to or more concentrated than 0.05 $mol dm^{-3}$ but less concentrated than 0.2 $mol dm^{-3}$ should be labelled health hazard. |
| Potassium iodide All solutions are low hazard. |
| Sodium thiosulfate All solutions are low hazard. |
Describe one relevant precaution, other than eye protection and a lab coat, that should be taken to keep the risk associated with the chemicals used to a minimum. Explain your answer.
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564
600
569
The production of hydrogen gas over time can be measured, and the data used to determine the charge of one mole of electrons, known as the Faraday constant, $F$.
(a) The volumes of hydrogen gas produced during the electrolysis process are recorded in the table.
Process the results to calculate the volume of hydrogen gas produced, in cm$^3$, and the charge passed, in coulombs, $C$.
charge (C) = current (A) x time (s)
The current was kept constant at 0.80A.
[Image_1: Table showing time/s, reading on burette 1/cm^3, volume of hydrogen gas produced/cm^3, charge passed/C]
[2]
(b) Plot a graph on the grid to show the relationship between volume of hydrogen gas produced and charge passed.
Use a cross (x) to plot each data point. Draw the straight line of best fit.
[Image_2: Graph grid for plotting]
[2]
(c) Do you think the results obtained in (a) are reliable? Explain your answer.
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(d) (i) The gradient of the line of best fit gives the volume of hydrogen gas produced per coulomb.
Use the graph to determine the gradient of the line of best fit.
State the co-ordinates of both points you used in your calculation.
co-ordinates 1 ................................................... co-ordinates 2 ..................................................
gradient = ................................... cm$^3$C$^{-1}$
[2]
(ii) Calculate the number of moles of hydrogen gas produced per coulomb.
If you were unable to obtain an answer for (d)(i), you may use the value 0.148 cm$^3$C$^{-1}$, but this is not the correct answer.
[The molar volume of gas = 24.0 dm$^3$ at room temperature and pressure.]
................................... mol C$^{-1}$ [1]
(iii) Use your answer to (ii) and the half-equation for the production of $H_2(g)$ to calculate a numerical value for the Faraday constant (the charge of 1 mole of electrons).
$2H^+(aq) + 2e^- → H_2(g)$
................................... C mol$^{-1}$ [1]
(e) (i) The graph below shows the relationship between volume of $H_2(g)$ produced at the cathode and time, in a similar experiment.
Draw a line on the graph to show the relationship between volume of $O_2(g)$ produced at the anode and time in this experiment.
[Image_3: Blank graph with axis labels for time/s and volume of gas produced/cm^3]
[1]
(ii) Suggest why the volume of $O_2(g)$ measured in this experiment might be less than that shown by your drawn line.
Assume that no gas is lost from leaks.
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[1]
(f) In these experiments, the pressure of the gas inside the burette is assumed to be atmospheric pressure, $P_{atm}$.
However, the presence of water vapour and the mass of the solution in the burette change the pressure of the gas to $P_{new}$.
The expression below shows the relationship between $P_{new}$ and $P_{atm}$.
$P_{new} = P_{atm} - 2.81 - (9.81 imes ext{height of solution in burette})$
(i) Use the expression to sketch a graph on the axes below to show the relationship between $P_{new}$ and the height of solution in the burette.
[Image_4: Blank graph for sketching relationship]
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(ii) State how $P_{new}$ changes the value of the Faraday constant calculated at $P_{atm}$ in (d)(iii).
Explain your answer.
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(g) A student’s teacher suggested it would be cheaper to use copper rather than platinum electrodes in the electrolysis of dilute sulfuric acid.
[Image_5: Table showing half-equation and $E^o$/V values]
Using the information in the table, suggest what effect, if any, the use of copper electrodes would have on the volume of gas produced at each electrode. Explain your answer.
cathode .................................................................................................................
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anode .................................................................................................................
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579
573
563
