AS & A Level Chemistry - 9701 Paper 3 2013 Summer Zone 4

(a) Method
Experiment 1
● Support the plastic cup in the 250 cm³ beaker.
● Fill the unlabelled burette with FB 1.
● Run 26.00 cm³ of FB 1 from the burette into the plastic cup.
● Record the temperature of FB 1, $T_1$, in the space below.

$T_1 = ............. \degree C$

● Fill the burette labelled FB 2 with FB 2.
● Run 4.00 cm³ of FB 2 from the burette into the plastic cup.
● Stir the mixture thoroughly and record in your table the maximum temperature of the solution.
● Empty the plastic cup, rinse thoroughly with water and shake dry.

Experiment 2
● Support the plastic cup in the 250 cm³ beaker.
● Run 22.00 cm³ of FB 1 from the burette into the plastic cup.
● Run 8.00 cm³ of FB 2 from the burette into the plastic cup.
● Stir the mixture thoroughly and record in your table the maximum temperature of the solution.
● Empty the plastic cup, rinse thoroughly with water and shake dry.

Experiments 3 – 7
● Repeat the experiment using 18.00, 14.00, 10.00, 6.00 and 2.00 cm³ of FB 1 respectively. Add sufficient FB 2 each time to make sure that the total volume remains 30.00 cm³.

For each of your seven experiments, record in the space below
● the volume of FB 1,
● the volume of FB 2,
● the maximum temperature of the solution,
● the temperature rise, $\Delta T$, where $\Delta T = \text{maximum temperature recorded} - T_1$.

(b) (i) On the grid opposite, plot the temperature rise, $\Delta T$, on the $y$-axis against the volume of FB 1 on the $x$-axis. The scale for $\Delta T$ should extend at least 2 \degree C above your greatest temperature rise.

(ii) Draw a straight line of best fit through the points where the values of $\Delta T$ are increasing. Draw a second straight line of best fit through the points where the values of $\Delta T$ are decreasing.

(iii) From your graph, determine the value of the volume of FB 1 where the two lines of best fit intersect.

volume of FB 1 = ................................................ cm³

(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate how many moles of hydrochloric acid are contained in the volume recorded in (b)(iii).

moles of HCl = ........................ mol

(ii) Calculate how many moles of sodium hydroxide would react completely with the number of moles of hydrochloric acid in (c)(i).

moles of NaOH = ..................... mol

(iii) Calculate the concentration of FB 2. Remember that the combined volume of FB 1 and FB 2 in each experiment was 30.00 cm³.

concentration of FB 2 = ...................... mol dm^-3

(d) A student decided to modify the experiment. The total volume of the solution was increased to 50 cm³ and temperature rises were recorded for 5, 10, 15, 20, 25, 30, 35, 40 and 45 cm³ of FB 2. The volumes were measured using a 50 cm³ measuring cylinder. Discuss how these changes would affect the accuracy with which the concentration of FB 2 could be determined.

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A second way to determine the concentration of an alkali is by volumetric titration. In this experiment you will first dilute the sample of FB 2 that you used in Question 1 and then titrate the diluted solution using hydrochloric acid.

$\text{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$

FB 2 is aqueous sodium hydroxide, NaOH.
FB 3 is 0.200 mol$\text{dm}^{-3}$ hydrochloric acid, HCl.

(a) Method

**Dilution of FB 2**
• Use the burette labelled FB 2 to transfer 25.00$\text{cm}^3$ of FB 2 into the 250$\text{cm}^3$ graduated (volumetric) flask, labelled FB 4.
• Make up the contents of the flask to the 250$\text{cm}^3$ mark with distilled water.
• Stopper the flask and mix the contents thoroughly. This solution FB 4.

**Titration**
• Rinse the unlabelled burette thoroughly with distilled water and then with a little FB 3. Fill this burette with FB 3.
• Use a pipette to transfer 25.0$\text{cm}^3$ of FB 4 into a conical flask.
• Add to the flask a few drops of the acid-base indicator provided.
• Titrate the alkali in the flask with the acid, FB 3.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

The rough titre is ....................... $\text{cm}^3$.

• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Record in a suitable form below all of your burette readings and the volume of FB 3 added in each accurate titration. Make certain that any recorded results show the precision of your practical work.

(b) From your titration results obtain a suitable value to be used in your calculation. Show clearly how you have obtained this value.

25.0$\text{cm}^3$ of FB 4 required .............. $\text{cm}^3$ of FB 3.

(c) (i) Calculate how many moles of HCl are contained in the volume recorded in (b).

moles of HCl = .................... mol

(ii) Hence, calculate how many moles of NaOH are contained in 25.0$\text{cm}^3$ of FB 4.

moles of NaOH = .................... mol

(iii) Calculate the concentration of the sodium hydroxide, FB 2.

concentration of FB 2 = .................... mol$\text{dm}^{-3}$

(a) FB 5, FB 6, FB 7 and FB 8 are aqueous solutions each of which contains a single cation and a single anion. Some of the ions present are listed below.
Fe²⁺ Pb²⁺ Zn²⁺ I^- OH^- SO₄^2-
By observing the reactions that occur when pairs of the solutions are mixed together, you will be able to identify which solution contains which of these ions.
• To a 1 cm depth of FB 6, FB 7 and FB 8 in separate test-tubes add FB 5 dropwise until in excess. Leave each test-tube to stand for a few minutes and note any changes.
• Use a 1 cm depth of each solution for the remaining tests.
• Record your observations in the following table.

(b) From your observations deduce which solution contains each of the following ions.
[Table_1]
[3]
(c) FB 9 is an aqueous solution containing either sulfate, SO₄^2-, or sulfite, SO₃^2-, ions. Describe how you would determine which ion is present.
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Carry out this test and record your observations and conclusion.
observations ..................................................................................................
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The anion in FB 9 is ......................................... .
[2]
(d) A student is given two different samples and told that each contains one of the following organic compounds.
ethanol ethanal ethanoic acid
Describe two chemical tests that the student could carry out on each sample to determine which organic compound is present in each sample. For each test, give the reagent(s) to be used and explain how the different observations would allow the identity of the sample to be determined.
first reagent ..................................................
observations ................................................................................................
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explanation ................................................................................................
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second reagent ................................................
observations ................................................................................................
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explanation ................................................................................................
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