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Homework Questions
Module 5
pH, Kw, Ka
Part 1 :
/36
Part 2 :
/7
Metacog :
/3
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Target for Improvement (select at least 1)
Target
1
Calculate pH, Ka and pKa
2
Write expressions for Kw, Ka
3
Identify chiral carbons in a molecule
Other
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Part 1 - Current Topic Content
1. Methanoic acid, HCOOH, is a weak organic acid
which occurs naturally in ants and stinging
nettles.
a) Use an equation for the dissociation of
methanoic acid to show what is meant by a
weak acid. (1)
b) A 1.5 x 10-2 mol dm-3 solution of HCCOH has [H+] = 1.55 x 10-3 mol dm-3.
i) Calcualte the pH of this solution and give one reason why the pH scale is a more
convienent measurement for measuring acid concentrations than [H +].
(2)
ii) Write the expression for Ka for methanoic acid.
(1)
iii) Calculate the values of Ka and pKa for methanoic acid.
(3)
iv) Estimate the percentage of HCCOH molecules that have dissociated in this
aqueous solution of methanoic acid.
(1)
2. The Ka values for three acids are shown in the table below.
a) What information is provided by Ka values?
(1)
b) Complete the table above by calculating the values of each pKa.
(3)
-3
c) The pH value of 0.0450 mol dm hydrochloric acid is different from that of
0.0450 mol dm-3 ethanoic acid. Calculate the pH values for these two acids.
Show all your working.
(5)
3. A student measured the pH of a solution of sodium hydroxide as 13.54 at 25 °C.
Kw = 1.0 x 10-14 mol2 dm-6 at 25 °C.
a) Write down the expression for the ionic product, Kw, for water.
(1)
-3
b) Calculate the concentration, in mol dm , of this solution of sodium hydroxide.
(2)
4. Methanoic acid, HCOOH, is an ant’s main defense mechanism, squirted at
potential intruders and injected into ‘ant bites’. The common name for methanoic
acid is formic acid, named from the Latin ‘formica’ which means ‘ant’.
A chemist collected the formic acid squirted by 20 ants and added sufficient water to
make 25.0 cm3 of a solution X.
The chemist titrated the solution X with sodium hydroxide, NaOH (aq).
● 20.0 cm3 of NaOH were required to neutralise the formic acid.
● The equation for the neutralisation of formic acid is shown below.
HCOOH(aq)
+ NaOH(aq)
→
HCOONa(aq) + H2O(l)
a) Write the ionic equation for this reaction.
(1)
b) Sodium hyroxide is a strong alkali. The concentration of NaOH(aq) used in
the titration was 0.00750 mol dm-3.
Calculate the pH of this solution. [Kw = 1.00 x 10-14 mol2 dm-6 ]
(3)
c) Calculate the amount in mol of HCOOH that was neutralised in the titration.(2)
d) An average ant contains 6.0x10-4 g of formic acid. Calculate the percentage
of a typical ant’s supply of formic acid collected by the chemist for the titration.
(3)
e) Formic acid is a weak acid with an acid dissociation constant, Ka, 1.6 x 10 -4
mol dm-3. HCOOH(aq)
⇌
H+(aq)
+
HCOO-(aq)
i) Write an expression for the acid dissociation constant, Ka, of formic acid.(1)
ii) The concentration of formic acid in solution X was 6.0 x 10 -3 mol dm-3
Calculate the pH of solution X.
(3)
f) The recommended treatment for an ant bite is the application of ‘bicarbonate
of soda’ which contains NaHCO3. Suggest, with the aid of an equation, how
NaHCO3 helps to relieve the effect of an ant bite.
(2)
g) Wasp stings are treated with vinegar. What does this suggest about the
nature of the active ingredient in a wasp sting? Explain your answer.
(2)
Part 2 - Interleaved Content - Chirality
1. Abiraterone, C24H31No, is a new drug under trial as a treatment for cancer of
the prostate gland. The structure of abiraterone is shown below.
a) A molecule of abiraterone is chiral because it has a number of asymmetric
carbon atoms. On the structural formula above, circle three of these
asymmetric carbon atoms.
(2)
b) A molecule with an asymmetric carbon atom exists as two stereoisomers.
Explain why these are different compounds.
(1)
2. Glutamic acid exists as two isomers.
a) State the type of isomerism shown by glutamic acid and explain why glutamic
acid can form two isomers.
(3)
b) Identify the structural feature present in glutamic acid that is responsible for
this type of isomerism by drawing a circle around it on the diagram below. (1)