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(a) (i) Write an equation for the thermal decomposition of magnesium nitrate(V). ................................................................................................................................................................. [1]
(ii) Calculate the mass of magnesium oxide and volumes of nitrogen(IV) oxide and oxygen produced under room conditions when 1 mole of magnesium nitrate(V) is heated. [Ar: O, 16.0; Mg, 24.3]
You should assume that one mole of any gas occupies 24.0dm³ under room conditions. [1]
(b) (i) Draw and label a diagram of the apparatus and experimental set-up you would use. The set-up needs to be capable of absorbing the nitrogen(IV) oxide and collecting the oxygen separately and in sequence. [4]
(ii) State the volume of the gas collector to be used to collect oxygen in (i). Calculate a mass of magnesium nitrate(V) to be heated that would produce a stated volume of oxygen appropriate for the collector. [Ar: N, 14.0; O, 16.0; Mg, 24.3]
You should assume that one mole of any gas occupies 24.0dm³ under room conditions. [1]
(c) List the measurements you would make when carrying out the experiment. .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. [3]
(d) (i) How could you make sure that the magnesium nitrate(V) had completely decomposed in the experiment? .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. [1]
(ii) To make sure that the volume of gas measured is accurate, what should you do before taking the measurement? .................................................................................................................................................................. .................................................................................................................................................................. [1]
(e) Explain how you would use the results of the experiment to confirm that the decomposition had occurred according to the molar ratios in the equation. .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. [2]
(f) What precautions would you take to make sure that the experiment could be performed safely? .................................................................................................................................................................. .................................................................................................................................................................. .................................................................................................................................................................. [1]
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An experiment was set up to investigate how the cell potential of a cell containing a metal, M, in contact with an aqueous solution of its ions, Mn+(aq) (where n = 1, 2 or 3), changed as Mn+(aq) was diluted. Since a standard hydrogen half-cell was not available, a standard half-cell consisting of silver in contact with a 1 mol dm−3 solution of silver ions was used to connect to the half-cell with M in contact with Mn+(aq).
The metal electrodes of the two half-cells were connected via a voltmeter, reading to two decimal places. This was used to measure the cell potential of the cell.
The cell potential was measured for various concentrations of Mn+(aq) and the results obtained are shown in the table below.
(a) Complete the third column of the table below. Give each answer to two decimal places.
[Table_1]
concentration of Mn+(aq)/mol dm−3 | cell potential / V | log [Mn+(aq)]
5.00 × 10−1 | 0.94 |
1.00 × 10−1 | 0.96 |
4.00 × 10−2 | 0.97 |
1.00 × 10−2 | 0.99 |
5.00 × 10−3 | 1.00 |
2.00 × 10−3 | 1.01 |
8.00 × 10−4 | 1.02 |
2.00 × 10−4 | 1.04 | [2]
(b) Plot a graph to show the relationship between log [Mn+(aq)] and the cell potential measured and draw the line of best fit.
[2]
(c) Are there any anomalous points on your graph? If so, circle those points. Give a reason for your answer. [2]
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(d) It is known that the cell potential of a cell, E, is related to the standard electrode potential, E°, by the equation:
$$E = E^o - \frac{0.06 \log [Mn+(aq)]}{n}$$
(i) Use your graph to determine the charge, n, of the Mn+ ions. Draw appropriate lines on your graph to enable you to calculate its slope and show in the space below, how n was calculated. [3]
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(ii) Use your graph to determine the standard electrode potential, E°, of the cell. [1]
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(e) The standard electrode potential for silver is +0.80 V. Calculate the standard electrode potential for the metal, M. Use the data given on page 12 to suggest the identity of M. [1]
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(f) Write an overall equation for the cell reaction which is taking place. [1]
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(g) The solutions contained in the two half-cells must be connected using a salt bridge.
(i) Why is a salt bridge necessary? [1]
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(ii) Which (if any) of the following salts would be suitable to use in the salt bridge: potassium chloride, potassium nitrate, potassium sulfate? If you consider any to be unsuitable, explain why. [2]
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