6 74 b use your graph from part a to determine the i

(6)
74

(b) Use your graph from part (a) to determine the
(i) volume of sodium hydroxide solution at the end-point of the titration
................... cm
3
(ii) volume of sodium hydroxide solution needed to neutralise half the acid
................... cm
3
(iii) pH of the half-neutralised mixture. Give your answer to one decimal place.
.........................
(3)
(c) Use the pH of the half-neutralised mixture from part (b) (iii) to calculate the value of the
acid dissociation constant, K
a
, of the acid Y . Show your working.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
(2)
(d) The table below shows the K
a
values for some organic acids.
Acid K
a
/ mol dm
–3
Methanoic acid 1.6 ×10
–4
Ethanoic acid 1.7 ×10
–5
Iodoethanoic acid 6.8 ×10
–4
Propanoic acid 1.3 ×10
–5
Use your answer from part (c) to identify acid Y from this table.
........................................................................................................................
(1)
75

(e) For the pipette and the burette, the maximum total errors are shown below. These errors
take into account multiple measurements.
pipette
burette
± 0.05 cm
3
± 0.15 cm
3
Estimate the percentage error in using each of these pieces of apparatus. You should use
your answer to part (b) (i) to estimate the percentage error in using the burette.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(1)
(f) Calculate the difference between the K
a
value from part (c) and the K
a
value of the acid
you identified as the acid Y in the table in part (d).
Express this difference as a percentage of the value given in the table in part (d). (If you
could not complete the calculation in part (c), you should assume that the K
a
value
determined from the graph is 1.9 ×10
–4
mol dm
–3
. This is not the correct value.)
........................................................................................................................
........................................................................................................................
(1)
(g) Other than by using a different pH meter, state one way in which the accuracy of the pH
readings could be improved.
........................................................................................................................
........................................................................................................................
(1)
(h) State why there was little change in the pH value of the mixture when between 8 cm
3
and
20 cm
3
of alkali were added.
........................................................................................................................
(1)
(Total 16 marks)
76

®
WORK SHEET 10a
To prepare a sample of aspirin
This experiment is likely to require at least two practical sessions.
The aim of this experiment is to prepare a sample of aspirin
Introduction
Aspirin is prepared by the acylation of salicylic acid (2-hydroxybenzenecarboxylic acid) using
ethanoic anhydride as the acylating agent.
The reaction can be represented as follows.
HOOCC
6
H
4
OH
+
(CH
3
CO)
2
O

HOOCC
6
H
4
OCOCH
3
+
CH
3
COOH
d salicylic aci ethanoic
dr d anhy i e
r aspi in ethanoic d aci
Aspirin (2-ethanoylhydroxybenzenecarboxylic acid) is an antipyretic drug (reduces fever by lowering
body temperature) and an analgesic (relieves pain).
Aspirin does not react in the acidic conditions in the stomach, but is hydrolysed in the alkaline
conditions found in the intestines to produce ethanoate ions and salicylate
(2-hydroxybenzencarboxylate) ions. Salicylates lower the body temperature of feverish patients and
have a mild analgesic effect relieving headaches and other pain. The toxic dose is relatively high, but
symptoms of poisoning can occur with quite small quantities.