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Edexcel Jan 2014 (IAL) Paper 4 Q18
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![18 (a) Ammonia is a weak alkali. Consider the following equilibrium that exists in an aqueous solution of ammonia. NH3(aq) + H2O(l) (cid:85) NH4 +(aq) + OH(aq) Kb = 1.8105 mol dm3 The base dissociation constant, Kb, for this reaction is( aq ][OH aq)] [NH (aq)] [NH 4 Kb (i) Calculate the concentration of hydroxide ions at equilibrium in a 4.0 mol dm3 aqueous solution of ammonia. Assume that the concentration of ammonia at equilibrium is 4.0 mol dm3 and that the equilibrium concentration of hydroxide ions is equal to the equilibrium concentration of ammonium ions. (ii) Calculate the pH of 4.0 mol dm3 ammonia solution. [Kw = 1.01014 mol2 dm6 at 298 K] (2) (2) 16 *P44451A01624*<br />
(b) (i) Calculate the pH of 4.0 mol dm3 hydrochloric acid, assuming it is fully ionized. (ii) Draw the titration curve, showing the change in pH when 4.0 mol dm3 hydrochloric acid is added to 25 cm3 of 4.0 mol dm3 ammonia solution, until 30 cm3 of the acid have been added. (1) (4) pH 121110987654321010 15 10 20 Volume of HCl(aq)/cm3 25 30 *P44451A01724* 17 Turn over<br />
*(iii) Select a suitable indicator for this titration, giving the colour change you would expect to see. Justify your selection. (3)(c) The titration above can be used to determine the concentration of ammonia solution when ammonia is distributed between the two immiscible solvents, trichloromethane and water. 18 An experiment is carried out to find the equilibrium constant for the reaction: NH3(CHCl3) (cid:85) NH3(aq) 75 cm3 of 4.0 mol dm3 aqueous ammonia solution and 75 cm3 of the trichloromethane are shaken together. The two liquids are allowed to separate and 25.0 cm3 of the aqueous layer is taken and titrated with 4.0 mol dm3 hydrochloric acid. The whole procedure is repeated. The average titre is 24.0 cm3. (i) Calculate the number of moles of ammonia, and hence the concentration of ammonia, in mol dm3, in the aqueous layer. (3) *P44451A01824*<br />
(ii) The initial volumes of the two solvents are the same. Hence deduce the concentration of ammonia in the trichloromethane layer in mol dm3. (1)(iii) Write the expression for the equilibrium constant, Kc, for this reaction and calculate its value. (1) (iv) Suggest why ammonia is much more soluble in water than in trichloromethane. (1)(Total for Question 18 = 18 marks) TOTAL FOR SECTION B = 50 MARKS *P44451A01924* 19 Turn over<br />
BLANK PAGE 20 *P44451A02024*<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJanuary_2014_IAL_QP_-_Unit_4_Edexcel_Chemistry_A-level_18-0.jpg)
![Reject Mark Reject Mark Acceptable Answers Question Number 18(a)(i) IGNORE sf except 1 If answer is 8.485 x 10-3 (moldm-3), award 2 marks If not, [OH-(aq)] = (Kb [NH3]) = (1.8 x 10-5 x 4.0) (1) = 8.485 x 10-3 (mol dm-3) (1) Acceptable Answers Question Number 18(a)(ii) IGNORE sf except 1 If answer is 11.9(3)/12, award 2 marks If not, EITHERMethod 1 [H+] = 1 x 10-14 [OH] = 1 x 10-14 (1) 8.485 x 10-3 = 1.179 x 10-12 ALLOW ecf from their answer to (i) pH = log 1.179 x 10-12 = 11.9(3) ALLOW ecf from their answer for [H+] (1) ORMethod 2 pOH = log 8.485 x 10-3 = 2.07 ALLOW ecf from their answer to (i) (1) pH = (142.07 =) 11.9(3) ALLOW ecf from their answer to pOH (1)<br />
Acceptable Answers Acceptable Answers Question Question Number Number 18(b)(ii) 18(b)(i) (pH = log 4.0) = 0.6(021) Reject Mark Mark 1 4 First mark graph starting at 11.9/ answer to a(ii),1 small square, provided above 7 (1) Second mark buffering region to 25 cm3 ALLOW any line showing a decrease in pH from 0 to 25 cm3 of HCl added (1) Third mark straight vertical portion between 8 and 1, midpoint below 7 and between 2 and 7 pH units long (1) Fourth mark finishing at +0.5 to 0.8, with at least 27.5 cm3 of HCl added ALLOW final pH as answer to (b)(i), within 1 pH unit, if pH is less than answer to (b)(i) or within 1 small square if pH is more than answer to (b)(i) (1) ALLOW If graph is drawn with aqueous ammonia added to hydrochloric acid, only the second and third marks are available for the correct vertical portion at 25cm3<br />
Reject Mark universal indicator loses all 3 marks litmus loses first mark only Acceptable Answers Question Number 18(b)*(iii) First mark any indicator from 4 to 10 or 12, 13 in the Data bookletsee end ALLOW ecf from the vertical portion on their graph (1) Second mark alkaline to acidic colour change for their stated indicator ALLOW acidic to alkaline colour change if their curve shows alkali added to acid (1) Third mark pH range (of indicator) is within the vertical section of the graph OR pKin ( 1) is in the vertical section of the graph OR pKin is nearest to the pH at the end/ equivalence point ALLOW indicator will change colour in the vertical section of the graph ALLOW Indicator will change colour at the end/ equivalence point ALLOW (because it is a) titration of a strong acid with a weak base (1)<br />
Acceptable Answers Question Number 18(c)(i) IGNORE sf except 1 If answer is 3.84 (mol dm-3), award 3 marks If not, number of moles of acid = 24.0 x 4 = 0.096 (1) 1000 EITHER number of moles ammonia = 0.096 in 25 cm3 (1) concentration of ammonia = 0.096 x 1000 25 = 3.84 (mol dm-3) (1) OR number of moles ammonia = 0.288 in 75 cm3 (1) concentration of ammonia = 0.288 x 1000 75 = 3.84 (mol dm-3) (1) IGNORE unit unless incorrect ALLOW ecf in both methods on their number of moles of ammonia Acceptable Answers Question Number 18(c)(ii) IGNORE sf except 1 (concentration of ammonia in trichloromethane =) 0.16 (mol dm-3) ALLOW ecf from their answer to (c)(i), provided it is less than 4.0 and given to 2 or more sf Mark Mark Acceptable Answers Question Number 18(c)(iii) Expression for Kc and answer needed for the mark Kc = [NH3(aq)] [NH3(CHCl3)] ALLOW one state symbol missing = 3.84 0.16 = 24(.0) IGNORE sf, including 1 sf, and units ALLOW ecf from answers to (c)(i) and (c)(ii) Reject Mark Kc expressions without both state symbols<br />
Acceptable Answers Reject Mark Question Number 18(c)(iv) Total for Question 18 = 18 marks (ammonia/ it is much more soluble in water) as can form hydrogen bonds with water ALLOW more/ stronger hydrogen bonds with water (than with trichloromethane) IGNORE answers based on polarity/ hydrophilic<br />](https://learnah.org/wp-content/uploads/2018/08/January_2014_IAL_MS_-_Unit_4_Edexcel_Chemistry_A-level_18-0.jpg)
