Form 6 AL CHEMISTRY
Assignment 9 – Acid-Base Equilibrium
1.
Given Ka for CH3(CH2)2COOH = 1.5 × 10-5 mol dm-3 at 298 K.
Calculate the pH of
(i) an aqueous solution of 0.10 M CH3(CH2)2COOH;
(ii) an aqueous solution of 0.05 M CH3(CH2)2COONa and 0.05 M CH3(CH2)2COOH; and
(iii) 1.0 dm3 of the solution in (ii) after the addition of 1.0 × 10-3 mol of solid NaOH. [94II 3(c)]
2.
Calculate the pH of a solution which is 0.05 M with respect to propanoic acid and 0.25 M with respect to sodium propanoate.
Given that Ka for propanoic acid is 1.34 × 10-5 mol dm-3. [86I 1(f)]
3.
Given that the equilibrium constant at room temperature for
H2S(aq) + 2 H2O(l) ↔ 2 H3O+(aq) + S2-(aq)
-20
2
-6
is 1.0 × 10 mol dm and that the saturated solution of H2S in water is 0.10 M, calculate the pH at which the sulphide ion
[81II 7(d)]
concentration will be 1.0 x 10-17 M.
4.
When excess solid Mg(OH)2 is shaken with 1.0 dm3 of 1.0 M NH4Cl solution, the resulting saturated solution has a pH of 9.0
at 298 K. The net equation for the reaction is
Mg(OH)2(s) + 2 NH4+(aq) ↔ Mg2+(aq) + 2 NH3(aq) + 2 H2O(l)
Calculate the concentration of OH- ion, NH3 and NH4+ ion, given that Kb for ammonia is 1.8 × 10-5 mol dm-3. [87II 3(a)]
5.
The pH of a 0.50 M solution of ammonia at 298 K is 11.48. Calculate the equilibrium constant at 298 K for the reaction :
NH3(aq) + H2O(l) ↔ NH4+(aq) + OH-(aq)
-14
2
-6
(At 298 K, Kw = 1.0 × 10 mol dm .)
[89I 2(b)]
*6. The following reversible reaction occurs in an aqueous solution of ammonium ethanoate :
CH3COO-(aq) + NH4+(aq) ↔ CH3COOH(aq) + NH3(aq)
(I)
(i) Write an expression for the dissociation constant, Ka , of ammonium ion.
(ii) Calculate the equilibrium constant of reaction (I) at 298 K.
(iii) For a 0.10 M solution of ammonium ethanoate at 298 K, calculate
(1) the concentration of ammonia, and
(2) the pH of the solution.
(At 298 K, the dissociation constants of ethanoic acid and ammonium ion are 1.76 × 10-5 mol dm-3 and 5.59 × 10-10 mol dm-3
respectively.)
[95II 2(c)]
7.
-
The dissociation constant of H2PO4 in water, Ka is 6.2 × 10-8 mol dm-3 at 298 K, where
+
[HPO 24 ( aq )][H ( aq )]
Ka =
[H 2 PO 4 ( aq )]
Calculate the pH at 298 K of
(i) an aqueous solution of 0.10 M NaH2PO4
(ii) an aqueous solution of 0.050 M NaH2PO4 and 0.050 M Na2HPO4.
(It may be assumed that the concentrations of H3PO4 and PO43- are negligible in these solutions.)
8.
The following solutions can act as a buffer :
(I) a solution with [CH3COOH] = 0.100 M
and [CH3COO-] = 0.075 M
(II) a solution with [CH3COOH] = 1.000 M
and [CH3COO-] = 0.750 M
(i) Calculate the approximate pH value of (I) and (II). [Ka = 1.85 × 10-5 mol dm-3 for CH3COOH]
(ii) Which of the above solution is the better buffer ? Explain your answer.
(iii) Can a solution of ethanoic acid alone act as a buffer ? Explain your answer.
[89II 2(a)]
9.
1 dm3 of a solution is obtained by mixing 500 cm3 of 0.2 M HCl and 500 cm3 of 0.7 M NaCN. Given that the dissociation
constant of HCN is 4.0 × 10-10 mol dm-3, calculate the pH of this solution.
[88II 1(b)]
10. A solution X is formed by mixing 0.50 dm3 of 0.30 M methanoic acid with 0.5 dm3 of 0.15 M sodium hydroxide.
(i) Given that Ka for methanoic acid is 1.75 × 10-4 mol dm-3 at 298 K, calculate the pH of solution X.
(ii) What special property does solution X have ? What is it commonly known as ?
[87II 3(b)]
11. When 0.050 mole of hydrogen chloride gas is passed into 1.0 dm3 of 0.10 M RCOONa solution the pH of the resulting
solution is found to be 6.0 at 298 K. Calculate
(1) Ka for the organic acid RCOOH.
(2) the pH of a 0.10 M RCOOH solution.
(3) the pH of the original 0.10 M RCOONa solution.
(Kw = 1.0 × 10-14 mol2 dm-6)
[85II 3(a)]
*12.Carbonic acid, H2CO3, is a polyprotic acid which ionizes in two steps.
(i) Using K1 and K2 to denote the ionization constants at the first and second ionizations respectively, write an ionizationconstant expression for each step.
(ii) Calculate the pH of a buffer solution obtained by adding 1.0 cm3 of 1.0 M sodium hydroxide solution to 100 cm3 of 1 0.1
M sodium hydrogencarbonate solution. You may assume that the total volume of the solutions is 100 cm3 and that the
sodium hydroxide added reacts quantitatively with sodium hydrogencarbonate.
[ For H2CO3 : K1 = 4.3 × 10-7 mol dm-3 and
K2 = 4.8 × 10-11 mol dm-3 at 298 K.]
[83IB 8(a)]
13. The pH of a 0.02 M solution of ethanedioic acid H2C2O4 is 1.80. Calculate the concentrations of each species present in the
solution at 25°C. (It may be assumed that the concentration of C2O42- is negligibly small in the solution.)
[Dissociation constants of ethanedioic acid at 25°C are :
K1 = 6.5 × 10-2 mol dm-3
K2 = 6.1 × 10-5 mol dm-3 ]
[90II 3(b)]
14. A weak base MOH has an ionization constant Kb = 2.0 × 10-5 mol dm-3, where Kb = [M+][OH-] / [MOH].
Solution S is made up of 0.10 mol of MOH in 1.0 dm3, and a second solution T has 0.10 mol of MOH and 0.50 mol of MCl in
1.0 dm3.
(i) Calculate the pH of solutions S and T.
(ii) 0.01 mol of a strong acid HX is added separately to the solutions S and T. The new value for solution S is 10.26;
calculate that for solution T. What conclusion can you draw from this result ?
(Kw = 1.0 × 10-14 mol2 dm-6)
[91II 2(c)]
*15.(i) Explain the difference between the equivalence point and the end point of a titration.
(ii) 40.0 cm3 of an aqueous solution of a weak acid, HA(aq), was titrated with a strong base, MOH(aq), at 298 K. The initial
pH, before the addition of base, was 2.70. At the equivalence point of the titration, the pH was 8.90. Calculate
(1) the initial concentration of the acid.
(2) the volume of the base added to reach the equivalence point.
(3) the concentration of the base.
(Ka for HA = 1.8 × 10-5 mol dm-3 ; Kw = 1.0 × 10-14 mol2 dm-6)
[92II 3(c)]
Answers
1. (i) 2.91
(ii) 4.82
(iii) 4.84
2. 5.57
3. 2.0
4. [OH-] = 1.0 × 10-5 M ; [NH3] = 0.357 M ; [NH4+] = 0.643 M
5. 1.8 × 10-5 M
6. (ii) 3.18 × 10-5
(iii) (1) 5.6 × 10-4 M
(2) 7.00
7. (i) 4.10
8. (i) 4.61
9. 9.80
10. (i) 3.76
11. (1) 1 × 10-6
12. (ii) 9.36
13. [H ] = 0.01585 M ; [OH ] = 6.31 × 10
14. (i) S 11.15
+
T 8.60
-
(ii) 8.55
-13
(ii) 7.21
(2) 3.5
(3) 9.5
-
M ; [HC2O4 ] = 0.016 M ; [H2C2O4] = 0.004 M
15. (1) 0.2212 M
(2) 37.90 cm3
(3) 0.2335 M