Skip to content
OCR A Jun 2017 Paper 1 Q21
Answers available below
Show answer
Question
16
17
18
19
20
21
22
23
24
25
![21 This question is about the properties and reactions of ethanoic acid, CH3COOH. Ethanoic acid is a weak acid with an acid dissociation constant, Ka, of 1.75105 mol dm3 at 25 C. 20 (a) A student uses a pH meter to measure the pH of a solution of CH3COOH at 25 C. The measured pH is 2.440. Calculate the concentration of ethanoic acid in the solution. Give your answer to three significant figures. concentration =mol dm3 [3] (b) Ethanoic acid is added to another weak acid, fluoroethanoic acid, FCH2COOH (Ka = 2.19103 mol dm3). An equilibrium is set up containing two acid-base pairs. Complete the equilibrium and label the conjugate acid-base pairs as A1, B1 and A2, B2. CH3COOH + FCH2COOH+[2]<br />
21 (c) The student plans to prepare a buffer solution that has a pH of 4.50. The buffer solution will contain ethanoic acid, CH3COOH, and sodium ethanoate, CH3COONa. The student plans to add 9.08 g CH3COONa to 250 cm3 of 0.800 mol dm3 CH3COOH. The student assumes that the volume of the solution does not change. (i) Show by calculation whether, or not, the students experimental method would produce the required pH. Show all your working. [5] (ii) When the student prepares the buffer solution, the volume of solution increases slightly. Suggest whether the pH of the buffer solution would be the same, greater than, or less than your calculated value in (c)(i). Explain your reasoning.[2]<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_QP_-_Paper_1_OCR_Chemistry_A_A-Level_21-0.jpg)
![H432/01 Mark Scheme June 2017 Question (a) 21 Answer FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 0.753, award 3 marks --------------------------------------------------------------------------------- [H+] = 10pH = 102.440 = 3.63103 (mol dm3)Marks [CH3COOH] = OR [H+]2 Ka (3.63103)2 1.75105= 0.753 (mol dm3)(b) CH3COOH2 + + FCH2COOCH3COOH + FCH2COOH B2 A1 A2 B1 OR B1 A2 A1 B2i.e. labels other way round 17 Guidance ALLOW use of HA and A ALLOW 3 SF up to calculator value of 3.630780548103 correctly rounded NOTE: Answer is same from unrounded [H+] calculator value and 3 SF [H+] value ALLOW 0.749 if [H+] has been subtracted from [CH3COOH] for greater accuracy at end 2 Watch for opposite order on RHS, i.e.: FCH2COO + CH3COOH2 Take great care matching labels ALLOW ECF for incorrect proton transfer as below. This is the ONLY ECF CH3COOH + FCH2COOH A1 B2 B1 A2 OR A2 B1 B2 A1 ECF i.e. labels other way round CH3COO + FCH2COOH2 + <br />
H432/01 Mark Scheme June 2017 Question (c) (i) [CH3COO] Answer Marks n(CH3COONa) = 9.08 82.0 OR 0.111(Calc: 0.1107317073) [CH3COO] = 9.08 82.0 1000 250 = 0.443 (mol dm3) OR n(CH3COOH) = 0.800= 0.200 (mol)250 1000 [H+]= Ka[CH3COOH] [CH3COO] OR Kan(CH3COOH) n(CH3COO) = 1.75105OR 1.751050.800 0.443 0.200 0.111 = 3.16105 (mol dm3)(must come from calculated [H+]) pH = log (3.16105) = 4.50[H+] pH --------------------------------------------------------------- LAST 3 marks are NOT available usingKa square root approach (weak acid pH)Kw /1014 approach (strong base pH) ------------------------------------------------------------------------------------ HendersonHasselbalch (HH) alternative pKa = log 1.75105 = 4.757 (or 4.756961951) [CH3COO] [CH3COOH] OR = pKalog [CH3COOH] [CH3COO] pH = pKa + log OR pKa + log 0.443 0.800 OR = pKalog 0.800 0.443 = pKa0.257= 4.7570.257 = 4.5018 Guidance ALLOW 2 sig fig ALLOW use of HA and A Mark by ECF ----------------------------------------------------------- Alternative method (If both methods are attempted, mark the method which produces the higher mark) [H+] [H+] = 10pH = 104.50 = 3.16105 (mol dm3)[CH3COO] [CH3COO] = Ka[CH3COOH] OR 1.751053.16105= 0.443 (mol dm3)mass of CH3COONa mass CH3COONa = 0.443250 1000 OR 0.1110.11182.0 = 9.08 (g)----------------------------------------------------------- Common errors 4.64 Use of M(CH3COONa) = 60 4 marks 2.40 Use of Ka of FCH2COOH 4 marks [H+] 0.800<br />
H432/01 Mark Scheme June 2017 Guidance M2 is dependent upon M1 ALLOW Change in [HA] and [A] is proportional Question (ii) pH is the same/constantAnswer Marks ratio/proportion [HA]/[A] is the sameTotal 12 19<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_MS_-_Paper_1_OCR_Chemistry_A_A-Level_21-0.jpg)
