AS & A Level Chemistry - 9701 Paper 3 2009 Winter Zone 2

This question concerns the solubility of FB1, potassium nitrate, in water.

The solubility of a substance in water is defined as:
the mass of substance that will dissolve in and just saturate 100 g of water at a particular temperature.
When a solution is saturated the dissolved solid is in equilibrium with undissolved solid.
When a solution of potassium nitrate is cooled it becomes saturated when crystals form in the solution.

You are to investigate how the solubility of FB1 in water varies with temperature.

You are provided with the following materials.
weighing bottle, labelled FB1, containing potassium nitrate
distilled water

Read through the instructions before starting any practical work.

Method
• Weigh an empty boiling-tube.
• Add the contents of the weighing bottle labelled FB1 to the weighed boiling-tube.
• Reweigh the boiling-tube and its contents.
• Record, in an appropriate form below, your weighings and the mass of FB1 used.

(a) Weighings

(b) Preparing a saturated solution
• Fill the burette with distilled water.
• Add 14.00 cm³ of distilled water from the burette to the weighed boiling-tube containing FB1.
• Use the clamp as a holder for the boiling-tube. Take care not to break the tube by clamping it too tightly.
• Warm the tube carefully, while stirring the contents with a thermometer, until all the solid has dissolved. (Take care that you do not break the thermometer bulb or the tube while stirring.)
• Keeping the tube in the clamp attach the clamp to a stand.
• Let the tube cool and continue to stir gently with the thermometer.
• Watch the solution carefully. Note and record (on the next page) the temperature at which you first notice crystals forming in the solution.
• If you are uncertain about the temperature when crystals first form, warm the tube again for a few moments and repeat the cooling.
• As soon as you have recorded the temperature add a further 2.00 cm³ of distilled water to the tube from the burette.
• Warm to redissolve the solid and cool as before.
• Note and record (on the next page) the temperature at which crystals now form in the solution. This will be lower than the temperature obtained with 14.00 cm³ of water.
• Repeat the addition of 2.00 cm³ of distilled water, the heating and the cooling, until you have four readings in total.

(c) In an appropriate form in the space below, record the following.
• the total volume of distilled water in the boiling-tube
• the temperature at which crystals first appeared for each solution
Make certain that your results show the precision of your working.

(d) For each solution, calculate the solubility (in grams of solid per 100 g of water) using the following formula.

\[ \text{solubility} = \frac{100}{\text{volume of water}} \times \text{mass of FB1 dissolved} \]

Complete the table below to show the solubility at different temperatures.
In all calculated values show appropriate significant figures.

solubility (in grams of solid per 100 g of water)	temperature / °C

(e) Plot solubility against temperature and draw an appropriate line through the points plotted. Do not start at zero on either axis. You will need to be able to find the solubility of FB1 at 42.5°C.

[Image of graph paper]

From the graph plotted the solubility of FB1 in water at 42.5°C is
.......... g of solid per 100 g of water. [6]

(f) Describe how the solubility of FB1 changes with temperature.
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(g) Use your answer to (f) and your understanding of equilibrium systems to explain if dissolving FB1, KNO₃, under equilibrium conditions is exothermic or endothermic.
$$ \text{KNO}_3(s) + \text{aq} \iff \text{KNO}_3(\text{aq}) $$
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(h) Suggest two possible sources of inaccuracy, other than poor experimental technique, in this experiment.
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2 ...............................................................................................................
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(i) A solution of KNO₃, saturated at 60°C, is prepared in a thermostatically controlled water bath.

The solubility of KNO₃ at 60°C can be calculated if the mass of the solution and the mass of solid dissolved in the solution can be determined. Suggest steps to enable you to find these masses.
You may not need all of these numbered steps.

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Show how you would calculate the solubility of KNO₃ at 60°C from the mass of the solution and the mass of solid dissolved in the solution. [2]

You are provided with three solids, FB 2, FB 3, and FB 4.
Each of the solids contains one cation from those on page 11 and a sulfite or sulfate anion.

You will carry out specified tests to identify the cations and anions present in FB 2, FB 3 and FB 4. Use the data on pages 11 and 12.

At each stage of any test you are to record details of the following.

• colour changes seen
• the formation of any precipitate
• the solubility of such precipitates in an excess of the reagent added

Where gases are released they should be identified by a test, described in the appropriate place in your observations.

You should indicate clearly at what stage in a test a change occurs.
Marks are not given for chemical equations.
No additional tests for ions present should be attempted.
If any solution is warmed a boiling-tube MUST be used.

(a) In separate boiling-tubes, dissolve half of each of the solids FB 2, FB 3 and FB 4 in a minimum volume of dilute hydrochloric acid.
Gently warm each of the boiling-tubes.
Add distilled water so that each boiling-tube is approximately $\frac{2}{3}$ full.
Record your observations in an appropriate form in the space below.
[2]

(b) The cations present in FB 2, FB 3 and FB 4 can be identified by reaction of each solution, with aqueous sodium hydroxide and with aqueous ammonia.
React 1 cm depth of each of the solutions prepared in (a) with each of these two reagents.
Record, in an appropriate form, in the space below your observations for these reactions.

Conclusions
Using your observations you should be able to identify the cation in two of the solutions.
For the remaining solution you should be able to identify two possible cations.
FB 2 contains the cation(s) ................................................ .
FB 3 contains the cation(s) ................................................ .
FB 4 contains the cation(s) ................................................ .
[5]

(c) Use the information on pages 11 and 12 to select a reagent to distinguish between the two possible cations identified as present in one of the solutions in (b).
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Carry out the test with the selected reagent.
observation ............................................................................
conclusion ..............................................................................
[1]

(d) In separate boiling-tubes shake the remaining half of each solid with 3 cm depth of distilled water.
If any solid does not readily dissolve in water filter the mixture and retain the solution formed.
You will need to keep some of the FB 2 solution for test (f).
Carry out the following tests.
test | observations
FB 2 | FB 3 | FB 4
To 1 cm depth of the solution in a test-tube, add 1 cm depth of aqueous barium chloride, then,
add 2 cm depth of dilute hydrochloric acid.
Draw appropriate conclusions as to the identity of the anion in each solution.
FB 2 contains the anion ...................................... .
FB 3 contains the anion ...................................... .
FB 4 contains the anion ...................................... .
[3]

(e) Explain why aqueous barium chloride must be added before hydrochloric acid when distinguishing between a sulfite and a sulfate.
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[1]

(f) Carry out the following test with the solution of FB 2 prepared in (d).
test | observation
To 1 cm depth of the solution of FB 2 in a test-tube add 1 cm depth of aqueous potassium iodide, then
add a few drops of starch solution.
What is the nature of the reaction taking place between FB 2 and potassium iodide?
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[2]