Question
Solid sodium sulfite is often provided as the hydrated salt, Na2SO3•xH2O, where x is an integer.
You will determine x by using a solution of this sodium sulfite and reacting it with an excess of aqueous iodine.
Na2SO3(aq) + I2(aq) + H2O(l) → Na2SO4(aq) + 2I–(aq) + 2H+(aq)
The amount of iodine remaining will be determined by titration using a known concentration of sodium thiosulfate, Na2S2O3.
I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq)
FA 3 is a solution containing 31.50 g dm–3 of hydrated sodium sulfite, Na2SO3•xH2O.
FA 4 is 0.100 mol dm–3 iodine, I2.
FA 5 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3.
FA 6 is starch indicator.
(a) Method
● Pipette 10.0 cm3 of FA 3 into a conical flask.
● Pipette 25.0 cm3 of FA 4 into the same flask.
● Swirl the flask to mix the contents.
● Fill the second burette with FA 5.
● Add FA 5 to the flask until the mixture is yellow.
● Add approximately 10 drops of FA 6.
● Complete the rough titration by adding FA 5 until the mixture is colourless.
● Record your burette readings in the space below.
The rough titre is …… cm3.
● Carry out as many accurate titrations as you think necessary to obtain consistent results.
● Make sure 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 5 added in each accurate titration.
I
II
III
IV
V
VI
VII
[7]
(b) From your accurate titration results, calculate a suitable mean value to use in your calculations.
Show clearly how you obtain the mean value.
10.0 cm3 of FA 3 plus 25.0 cm3 of FA 4 required …… cm3 of FA 5. [1]
(c) Calculations
(i) Give your answers to (c)(ii), (iii) and (iv) to an appropriate number of significant figures.
[1]
(ii) Use your answer to (b) to calculate the amount, in mol, of sodium thiosulfate, FA 5, required to react with the excess iodine which remained in the conical flask.
amount of Na2S2O3 = …… mol
Hence calculate the amount, in mol, of iodine, FA 4, remaining in the conical flask.
amount of I2 remaining = …… mol
[1]
(iii) Calculate the amount, in mol, of iodine, FA 4, added to the conical flask.
amount of I2 added = …… mol
Hence calculate the amount, in mol, of iodine that reacted with the 10.0 cm3 of sodium sulfite,
FA 3.
amount of I2 that reacted with Na2SO3 = …… mol
[1]
(iv) Use your final answer to (c)(iii) and the information on page 5 to calculate the amount, in mol, of sodium sulfite present in 1.00 dm3 of FA 3.
amount of Na2SO3 in 1.00 dm3 = …… mol [1]
(v) Use your answer to (c)(iv) to calculate the value of x in Na2SO3•xH2O.
x = …… [2]
(d) A student suggests that sodium carbonate should be added to each mixture of sodium sulfite and iodine in the conical flask before titrating with sodium thiosulfate.
State whether you agree with the student. Explain your answer.
. ……
. …… [1]
[Total: 15]
Qualitative analysis
For each test you should record all your observations in the spaces provided.
Examples of observations include:
● colour changes seen
● the formation of any precipitate and its solubility (where appropriate) in an excess of the reagent added
● the formation of any gas and its identification (where appropriate) by a suitable test.
You should record clearly at what stage in a test an observation is made.
Where no change is observed you should write ‘no change’.
Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given.
If any solution is warmed, a boiling tube must be used.
Rinse and reuse test-tubes and boiling tubes where possible.
No additional tests should be attempted.
3
FA 7 and FA 8 are solutions containing a total of three cations and two anions. Two of the cations and both of the anions are listed in the Qualitative analysis notes.
(a) (i) Carry out the following tests and record your observations. Use a fresh 1 cm depth of solution in a test-tube for each test.
observations test
FA 7 FA 8
Test 1
Add a 1 cm depth of dilute nitric or hydrochloric acid and allow to stand for 2 minutes, then
add a few drops of aqueous barium nitrate or aqueous barium chloride.
Test 2
Add a few drops of acidified aqueous potassium manganate(VII).
Test 3
Add a few drops of aqueous iron(III) chloride and allow to stand for 1 minute.
[4]
(ii) From your test results, give the formulae of the anions present in FA 7 and FA 8. If the tests do not allow you to positively identify an anion, write ‘unknown’.
anion in FA 7 = ……
anion in FA 8 = ……
[2]
(b) (i) Select reagents for tests to identify as many of the cations as possible in FA 7 and FA 8.
Carry out your tests and record your reagents, conditions and observations.
[4]
(ii) From your test results, give the formulae of as many cations as possible in FA 7 and FA 8.
If the tests do not allow you to positively identify a cation, write ‘unknown’.
FA 7 contains …… .
FA 8 contains …… .
[2]
(iii) Write an ionic equation for one reaction you observed in (b)(i). Include state symbols.
. …… [1]
[Total: 13]
Qualitative analysis notes
1 Reactions of cations
cation reaction with
NaOH(aq) NH3(aq) aluminium, Al 3+(aq) white ppt. soluble in excess white ppt. insoluble in excess ammonium, NH4+(aq) no ppt. – ammonia produced on warming barium, Ba2+(aq) faint white ppt. is observed unless no ppt.
[Ba2+(aq)] is very low calcium, Ca2+(aq) white ppt. unless [Ca2+(aq)] is very no ppt. low chromium(III), Cr3+(aq) grey-green ppt. soluble in excess grey-green ppt. insoluble in excess giving dark green solution copper(II), Cu2+(aq) pale blue ppt. insoluble in excess pale blue ppt. soluble in excess giving dark blue solution iron(II), Fe2+(aq) green ppt. turning brown on green ppt. turning brown on contact with air contact with air insoluble in excess insoluble in excess iron(III), Fe3+(aq) red-brown ppt. insoluble in excess red-brown ppt. insoluble in excess magnesium, Mg2+(aq) white ppt. insoluble in excess white ppt. insoluble in excess manganese(II), Mn2+(aq) off-white ppt. rapidly turning brown off-white ppt. rapidly turning brown on contact with air on contact with air insoluble in excess insoluble in excess zinc, Zn2+(aq) white ppt. soluble in excess white ppt. soluble in excess
2 Reactions of anions
anion reaction carbonate, CO32– CO2 liberated by dilute acids chloride, Cl –(aq) gives white ppt. with Ag+(aq) (soluble in NH3(aq)) bromide, Br –(aq) gives cream / off-white ppt. with Ag+(aq) (partially soluble in NH3(aq)) iodide, I–(aq) gives pale yellow ppt. with Ag+(aq) (insoluble in NH3(aq)) nitrate, NO3–(aq) NH3 liberated on heating with OH–(aq) and Al foil nitrite, NO2–(aq) NH3 liberated on heating with OH–(aq) and Al foil; decolourises acidified aqueous KMnO4 sulfate, SO42–(aq) gives white ppt. with Ba2+(aq) (insoluble in excess dilute strong acids); gives white ppt. with high [Ca2+(aq)]
sulfite, SO32–(aq) gives white ppt. with Ba2+(aq) (soluble in excess dilute strong acids); decolourises acidified aqueous KMnO4 thiosulfate, S2O32–(aq) gives off-white / pale yellow ppt. slowly with H+
Mark scheme
2(a) I The following data must be shown: 1
• burette readings and titre for rough titration
• 2 × 2 ‘box’ showing both accurate burette readings.
II Headings and units correct for accurate titration table and headings match readings. 1
• initial / start AND (burette) reading / volume + unit
• final / end AND (burette) reading / volume + unit
• titre OR volume / FA 4 AND used / added + unit
III All accurate burette readings given to the nearest 0.05 1
IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1
Accuracy marks
Check and correct titre subtractions where necessary. Exclude any titre from the calculation for the mean where final burette
reading is greater than 50(.00). Examiner selects the best titres for calculating the mean, using the hierarchy:
2 identical titres, titres within 0.05 cm3, titres within 0.10 cm3 etc.
Examiner subtracts (corrected) candidate’s titre from Supervisor’s titre.
Write and ring Supervisor’s value next to the accurate titration table of each candidate, also candidate mean value
(calculated by examiner) and δ.
Award V if δ ⩽ 0.50 cm3 3
Award VI if δ ⩽ 0.30 cm3
Award VII if δ ⩽ 0.20 cm3
2(b) Candidate must average two (or more) titres that are all within 0.20 cm3 1
AND
give the answer to 2 dp.
AND
working must be shown or ticks must be put next to the two (or more) accurate titres selected.
2(c)(i) All final answers for (c)(ii), (c)(iii), (c)(iv) are to 3–4 sf 1
2(c)(ii) Correctly calculates 1
amount of (n) Na2S2O3 = 0.1 × (b) / 1000 mol
AND
n(I2) = ans / 2
2(c)(iii) Correctly calculates 1
initial amount of (n) I2 = 2.5(0) × 10−3 mol
AND
n(I2) that reacted = 2.5(0) × 10−3 – final answer to (ii)
2(c)(iv) Correctly uses 1
amount of (n) Na2SO3 = final answer to (iii) × 100 mol
2(c)(v) M1 Correctly uses 1
Mr Na2SO3•xH2O = 31.5(0) / (c)(iv)
AND
Mr Na2SO3 = 126.1
M2 n(H2O) = (answer above – 126.1)/18 1
AND
answer is a (correctly rounded) integer
OR
M1 mass(Na2SO3) = (c)(iv) × 126.1
AND
mass(H2O) = 31.5(0) – answer above
M2 n(H2O) = mass above / 18
AND
x = n(H2O) / n(Na2SO3)
AND
answer is a (correctly rounded) integer
2(d) agree: Na2CO3 would neutralise acid (formed in reaction between sulfite and iodine which would react with the thiosulfate) / 1
some thio would (otherwise) react with the acid (formed)
OR
disagree: (no advantage) as the reaction of thio with acid is slow
OR
disagree: (no advantage) as Na2CO3 will react with iodine (so less to react with thio)
FA 7 is Na2S2O3; FA 8 is Al NH4(SO4)2(aq)