Edexcel Jan 2017 (IAL) Paper 2 Q18 (with explained solutions) |

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Edexcel Jan 2017 (IAL) Paper 2 Q18

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 18 Sulfur dioxide, SO2, is added to commercial wine to act as an antioxidant and inhibitor of microbial growth. The SO2 that is dissolved in the wine is called free SO2. However, there is also SO2 combined with compounds in the wine; this bound SO2 can be released by the addition of sodium hydroxide. The free SO2 and bound SO2 give the total SO2 present in the wine. The amount of sulfur dioxide can be determined by titration with an iodine solution. The equation is SO2 + I2 + 2H2O (cid:111) 2I + 4H+ + SO4 2 Free SO2 determination A 50.00 cm3 sample of white wine was pipetted into a conical flask. The sample was acidified with 5 cm3 of dilute sulfuric acid and 2 cm3 of starch solution was added. This mixture was titrated with 0.00100 mol dm3 iodine solution. The end-point was taken when a dark blue colour persisted for about two minutes. The procedure was repeated and the following burette readings obtained. Final volume / cm3 Initial volume / cm3 Volume added / cm3 25.60 10.00 15.60 41.20 25.60 15.60 (a) (i) Calculate the number of moles of iodine added in each 15.60 cm3 titre. 10 (ii) Hence deduce the number of moles of free SO2 in 50 cm3 of the wine. (iii) Calculate the concentration in mg dm3 of free SO2 in the wine. This is equivalent to the concentration in ppm. Give your answer to three significant figures. *P48368A01024* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A (1) (1) (2)<br />
A E R A S I H T N I E T I R W T O N O D Total SO2 determination Another 50.00 cm3 sample of the white wine was pipetted into a conical flask. To release the bound SO2, excess sodium hydroxide was added. After swirling, the mixture was left to stand for 15 minutes. This mixture was acidified with 10 cm3 of dilute sulfuric acid and 2 cm3 of starch solution was added. This mixture was then titrated with 0.00100 mol dm3 iodine solution. The following burette readings were obtained. Final volume / cm3 Initial volume / cm3 Volume added / cm3 23.40 0.00 23.40 (iv) Calculate the concentration in mg dm3 (ppm) of total SO2 in the wine. A E R A S I H T N I E T I R W T O N O D (v) Calculate the percentage uncertainty in the volume measured using the burette in the determination of both the free SO2 and the total SO2. Each time a burette is read the uncertainty is 0.05 cm3. Also calculate the percentage uncertainty in measuring 50 cm3 using a 50 cm3 pipette. Each time the pipette is used the uncertainty is 0.10 cm3. (1) (2) A E R A S I H T N I E T I R W T O N O D Burette uncertainty for free SO2Burette uncertainty for total SO2Pipette uncertainty*P48368A01124* 11 Turn over<br />
(vi) State in which of these SO2 determinations you have the greater confidence. Justify your answer. (1)D O N O T W R I T E I N T H I S A R E A (b) The recommended total concentration of SO2 in wine is 2040 ppm but the European Union has set a legal limit of 200 ppm for white wines. Suggest one possible problem with the total concentration of SO2 in white wine being outside each of these limits. (2) Less than 20 ppmMore than 200 ppm(c) Suggest why it is more difficult to get an accurate titre with red wine, using the method described above. (1)12 *P48368A01224* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br />
(d) The dot and cross diagram for sulfur dioxide can be drawn as shown. The sulfur atom has expanded its octet by having ten electrons in the outer energy level. xxxx xx(i) Draw the dot and cross diagram for sulfur dioxide which shows the sulfur atom with only eight electrons in its outer energy level. Use dots to represent the oxygen electrons and crosses to represent the sulfur electrons. (ii) A sulfur atom can expand its octet but an oxygen atom cannot. Suggest why this is so. (1) (1)(iii) Deduce the shape of the sulfur dioxide molecule and suggest the OSO bond angle. (2) ShapeBond angle(Total for Question 18 = 15 marks) A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P48368A01324* 13 Turn over<br />

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Reject Mark Reject Mark 1.6 x 10-5 /0.000016 Reject Mark Section B Question Number 18(a)(i) Acceptable Answers (Iodine n=0.00100 x 0.01560 =) 1.56 x 10-5 /0.0000156 (mol) Acceptable Answers (1:1 ratio so Free SO2 n=) 1.56 x 10-5 /0.0000156 (mol) Acceptable Answers Question Number 18(a)(ii) Question Number 18(a)(iii) If final answer 20.0 (ppm) then with or without working award (2) There are 3 operations in the calculation:0.050 or (50 1000) or x 20x 64.1/64x1000 One correct operation scores 1 mark. Two acceptable routes EITHER (c=1.56 x 10-50.050=)3.12 x 10-4/0.000312 (mol dm-3) (1) (Free SO2 ppm = 3.12 x 10-4 x 64.1 x 1000 = 19.999) = 20.0 (ppm or mg dm3) (1) OR m =(1.56 x 10-5 x 64.1=)9.996 x 10-4/0.0009996 (1) c =((9.996 x 10-40.05) x 1000=19.9992=) = 20.0 (ppm or mg dm3) (1) TE ans to (a)(ii)0.050 x 64.1 x 1000 Answer to 3 s.f. without working scores (2) ALLOW Use of 64, in place of 64.1<br />
Reject Mark Reject Mark Question Number 18(a)(iv) Question Number 18(a)(v) Acceptable Answers If final answer 30.0 (ppm) then with or without working award (1) EITHER Total SO2 ppm (= 23.4 x 20 =) 30 (ppm or mg dm3) 15.6 OR (Iodine n and total SO2 = 0.00100 x 0.0234) 2.34 x 10-5 /0.0000234 (mol) c=2.34 x 10-50.050 = 4.68 x 10-4 /0.000468 (mol dm-3) Total SO2 ppm = (4.68 x 10-4 x 64.1 x 1000 =) = 29.9988 / 30.0 (ppm or mg dm3) IGNORE SF TE ans to (a)(iii) x (23.4 15.6) Acceptable Answers One correct answer scores 1 Three correct answer scores 2 Uncertainty of burette titre for free SO2 result = ((2 x 0.05)15.60) x 100=) 0.64 (%) (1) OR Uncertainty of burette titre for Total SO2 result = ((2 x 0.05)23.40) x 100=) 0.43/0.427(%) (1) OR Pipette uncertainty = ((0.1050) x 100=) 0.20 (%) (1) Ignore SF<br />
Acceptable Answers Reject Mark Greater confidence in the free SO2 result because of the repeat OR Greater confidence in the total SO2 result because of the lower uncertainty (of the burette reading) Bound SO2 Acceptable Answers Reject Mark (less than 20 ppm) the level of SO2 is too low to inhibit microbial growth/oxidation (1) (more than 200 ppm) the taste of the wine is affected/taste becomes acidic/ at this level the SO2 is toxic/poisonous/harmful (1) Just acidic/low pH Acceptable Answers Reject Mark (Red colour from the wine likely to make) the dark blue colour/colour change/end-point/titre difficult to judge ALLOW Colour will make it hard to see (the colour of) the indicator. Compounds in the red wine may interact with the iodine/SO2/starch (to give an incorrect result) IGNORE References to alcohol content colour in burette Iodine/brown colour makes it difficult to see colour change Question Number 18(a)(vi) Question Number 18(b) Question Number 18(c)<br />
Question Number 18(d)(i) Acceptable Answers Either of the following diagrams Reject Mark Allow electron pairs to be on the same horizontal line Lone pairs of electrons do not have to be paired Acceptable Answers Reject Mark Sulfur has (3)d orbital(s) (that can be occupied)/ Oxygen is in Period 2 and has no (available) d orbitals ALLOW Sulfur has (3) d subshell that can be occupied Oxygen is in period 2 and has not available d subshell 2d/4d Promotion to s/p orbitals d shell Reject Mark Question Number 18(d)(ii) Question Number 18(d)(iii) Acceptable Answers Bent / V-shaped / ALLOW non-linear/angular Shown on a suitable diagram (1) 120 2.5 (1) TOTAL FOR QUESTION 18 = 15 MARKS<br />

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