AS & A Level Chemistry - 9701 Paper 3 2016 Summer Zone 3

You will determine the concentration of a solution of hydrochloric acid by diluting it and then titrating the diluted solution against an alkali.

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

FA 1 was made by dissolving 1.06 g of sodium hydroxide, NaOH, in distilled water to make 250 cm³ of solution.
FA 2 is hydrochloric acid, HCl.
bromophenol blue indicator

(a) Method

• Pipette 25.0 cm³ of FA 2 into the 250 cm³ volumetric flask. Keep remaining FA 2 for use in Question 2.
• Add distilled water to make 250 cm³ of solution and shake the flask thoroughly. Label this solution FA 3.
• Fill the burette with FA 3.
• Use the second pipette to transfer 25.0 cm³ of FA 1 into a conical flask.
• Add about 10 drops of bromophenol blue.
• Perform a rough titration and record your burette readings in the space below. The end point is reached when the solution becomes a permanent yellow colour.

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

Metal carbonates react with dilute acids to produce carbon dioxide. You will identify the metal, M, in a metal carbonate, $M_2CO_3$, by measuring the volume of carbon dioxide produced during the reaction of $M_2CO_3$ with excess hydrochloric acid.

$M_2CO_3(s) + 2HCl(aq) \rightarrow 2MCl(aq) + CO_2(g) + H_2O(l)$

FA 2 is hydrochloric acid, HCl, as used in Question 1.
FA 4 is $M_2CO_3$.

(a) Method

Read all instructions before starting your practical work. The diagrams below may help you in setting up your apparatus.



- Fill the tub with water to a depth of about 5 cm.
- Fill the 250 cm³ measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.
- Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub.
- Use the 50 cm³ measuring cylinder to place 50 cm³ of FA 2 into the reaction flask, labelled X.
- Check that the bung fits tightly in the neck of flask X, clamp flask X, and place the end of the delivery tube into the inverted 250 cm³ measuring cylinder.
- Weigh the container with FA 4 and record the mass in the space below.
- Remove the bung from the neck of the flask. Tip all the FA 4 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents.
- Swirl the flask occasionally until no more gas is evolved. Replace the flask in the clamp.
- Reweigh the container and record the mass, and the mass of FA 4 used, in the space below.
- When no more gas is collected, measure and record the final volume of gas in the measuring cylinder in the space below.

(b) Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Use the volume of gas you collected to calculate the number of moles of gas produced. [Assume that 1 mole of gas occupies 24.0 dm³ under these conditions.]

moles of gas = ................... mol

(ii) Use your answer to (i) to deduce the number of moles of $M_2CO_3$ used in the reaction.

moles of $M_2CO_3$ = ................... mol

(iii) Use your answer to (ii) and the mass of FA 4 used to calculate the relative formula mass, $M_r$, of $M_2CO_3$.

$M_r$ of $M_2CO_3$ = ...................

(iv) Use your answer to (iii) and the Periodic Table on page 12 to identify metal M. Explain your answer.

M is ..................................................
..............................................................................................................................
.............................................................................................................................. [4]

(c) (i) A 250 cm³ measuring cylinder can be read to ±1 cm³.
Calculate the maximum percentage error in your reading of the volume of gas.

maximum percentage error = ................... %

(ii) It is likely that the volume of carbon dioxide that you collected was less than the theoretical volume.
Give two reasons why this volume is likely to be less than the theoretical volume.
In each case, suggest and explain a modification to the practical procedure that could help to reduce the difference in volume.

reason ..................................................................................................................
..............................................................................................................................
modification ..................................................................................................................
..............................................................................................................................
reason ..................................................................................................................
..............................................................................................................................
modification ..................................................................................................................
.............................................................................................................................. [5]

(a) FA 5, FA 6, FA 7 and FA 8 are aqueous solutions of organic compounds. All of FA 5, FA 6, FA 7 and FA 8 contain carbon, hydrogen and oxygen only.
Half fill the 250 $cm^3$ beaker with water and heat it to about 80°C. Turn off the Bunsen burner. This will be used as a water bath.
To a 2 cm depth of aqueous silver nitrate in a boiling tube add 2 drops of aqueous sodium hydroxide and then add ammonia dropwise until the brown solid just disappears. This solution is Tollens’ reagent and is needed in a test in (i).

(i) Carry out the following tests on FA 5, FA 6, FA 7 and FA 8 and record your observations in the table.

| test | observations |
|------|--------------|
| | FA 5 | FA 6 | FA 7 | FA 8 |
| To a 1 cm depth in a test-tube, add a small spatula measure of sodium carbonate. | | | | |
| To a few drops in a test-tube, add a 1 cm depth of Tollens’ reagent.
Place the tube in the water bath and leave to stand.
When you have completed this test rinse all tubes used. | | | | |
| To a 1 cm depth in a test-tube, add a few drops of acidified potassium manganate $(VII)$.
Place the tube in the water bath and leave to stand. | | | | |

(ii) Using your observations from the table, what functional group is present in both FA 5 and FA 6?
....................................................................................................................

(iii) Using your observations from the table, what functional group is present in both FA 5 and FA 8?
....................................................................................................................

(iv) What type of reaction is occurring in the potassium manganate $(VII)$ test?
....................................................................................................................

(v) Using your observations from the table, what functional group is present in FA 7?
....................................................................................................................

(vi) Suggest a test that would confirm the presence of the functional group in a pure sample of FA 7. Include the result you would expect the test to give.
Do not carry out this test.
....................................................................................................................
....................................................................................................................
....................................................................................................................

(b) FA 9 and FA 10 are solids that each contain one anion from those listed in the Qualitative Analysis Notes on page 11.

(i) Carry out the following tests on FA 9 and FA 10 and record your observations in the table.

| test | observations |
|------|--------------|
| | FA 9 | FA 10 |
| To a spatula measure of solid in a boiling tube, add a 1 cm depth of aqueous sodium hydroxide. Warm, then, | | |
| add a small piece of aluminium foil. | | |
| Place a spatula measure of solid in a hard-glass test-tube. Heat gently at first and then more strongly. | | |

(ii) Using your observations from the table, which two anions could be present in FA 9 and FA 10?
anion .................................................... or ....................................................

(iii) Suggest a test that would allow you to decide which of the anions is present. State what observations you would expect.
....................................................................................................................
....................................................................................................................

(iv) Carry out this test on FA 9 and FA 10 to decide which anion is present in each.
observation for FA 9 .................................... anion in FA 9 is ...................................
observation for FA 10 ................................. anion in FA 10 is ...................................