Buffer solutions are solutions which resist to changes in pH. Usually, buffer solutions consist of a — Analytical Chemistry Chemistry Question
Buffer solutions
Buffer solutions are solutions which resist to changes in pH. Usually, buffer solutions consist of a weak acid and its conjugate base (for example CH3COOH / CH3COO) or a weak base and its conjugate acid (for example NH3/NH4+).
A buffer solution is formed by partial neutralization of a weak acid with a strong base or of a weak base with a strong acid. Alternatively, buffer solutions can be prepared by mixing the precalculated concentrations of each of the constituents.
The pH of a buffer solution, which is composed of a weak acid HA and its conjugate base A is calculated by the Henderson–Hasselbalch equation:
pH = pKa + log([A-]/[HA])
where Ka is the acid dissociation constant of the weak acid HA and [HA] and [A] are the standard relative concentrations of HA and A in the buffer solution, respectively.
Calculate the pH of a buffer solution which contains formic acid (Ka = 2.1×104 , c = 0.200 mol dm –3 ) and sodium formate (c = 0.150 mol dm –3 ).
Model Answer
The equilibrium, which governs the concentration of H+ within the solution is
HCOOH ⇄ HCOO− + H+
Hence [H+][HCOO−] / [HCOOH] = Ka = 2.1×10–4
since [HCOOH] ≈ 0.200 and [HCOO−] ≈ 0.150
[H+] = 2.1×10–4 × 0.200 / 0.150 = 2.8×10–4
and pH = 3.55.
Calculate the change in pH of the buffer solution in task 22.1 when 0.01000 mol of sodium hydroxide is added to the solution.
Model Answer
Since sodium hydroxide reacts with formic acid:
HCOOH + OH− ⇄ HCOO− + H2O
the concentration of formic acid in the solution is reduced to
[HCOOH] = 0.200 – 0.0100 = 0.190
and the concentration of formate is increased to
[HCOO−] = 0.150 + 0.0100 = 0.160
Therefore: [H+] = 2.1×10–4 × 0.190 / 0.160 = 2.5×10–4
pH = 3.60
Note that the addition of sodium hydroxide, which is a strong base, causes a very small increase of the pH of the solution.
Calculate the volume of sodium hydroxide swolution (c = 0.200 mol dm –3 ) which must be added to 100.0 cm 3 of acetic acid solution (CH3COOH, Ka = 1.8×10 –5 , c = 0.150 mol dm –3 ) in order to prepare a buffer solution with pH = 5.00.
Model Answer
Let V is the volume of the solution of sodium hydroxide. Therefore, the final volume of the solution will be (100.0 + V) cm 3 and the number of mmol of CH3COOH and OH− which are mixed are 100.0 cm 3 × 0.150 mmol cm –3 = 15.00 mmol and V cm 3 × 0.200 mmol cm –3 = 0.200×V mmol, respectively. From the reaction:
CH3COOH + OH− → CH3COO− + H2O
it is obvious that the amount of acetate produced is 0.200 × V mmol and the amount of acetic acid which remains unreacted is (15.00 – 0.200×V) mmol.
Hence, the concentration of each constituent in the buffer solution is:
[CH3COOH] = (15.00 – 0.200×V) / (100.0 + V) and
[CH3COO−] = 0.200 × V / (100 + V)
From the acid dissociation constant expression of acetic acid
[CH3COO−][H+] / [CH3COOH] = Ka = 1.8×10–5
It can be derived
[CH3COO−] / [CH3COOH] = Ka / [H+]
and
(0.200 × V) / (100 + V) / ((15.00 – 0.200×V) / (100.0 + V)) = 1.8×10–5 / 1.0×10–5
from which V = 48.21 cm–3.
The pH of a buffer solution containing benzoic acid (C6H5COOH, Ka = 6.6×105 , c = 0.0100 mol dm –3 ) and sodium benzoate (c = 0.0100 mol dm –3 ) is:
a) 5.00, b) 4.18, c) 9.82, d) 9.00.
Based on calculation choose the correct answer.
Model Answer
a
In the problems below equal volumes of the following solutions A and B are mixed:
Solution A: CH3COOH (Ka = 1.8×105 , c = 0.100 mol dm –3 ),
Solution B: NaOH (c = 0.0500 mol dm –3 )
(i) The final solution:
a) contains a weak acid; b) contains a strong base; c) is a buffer solution; d) none of the above.
(ii) The pH of the final solution is:
a) 3.02, b) 4.74, c) 3.17, d) 7.00
Model Answer
(i) c, (ii) b
In the problems below equal volumes of the following solutions A and B are mixed:
Solution A: CH3COOH (Ka = 1.8×105 , c = 0.100 mol dm3 ),
Solution B: NaOH (c = 0.150 mol dm3 )
(i) The final solution:
a) contains a weak acid; b) contains a strong base; c) is a buffer solution; d) none of the above
(ii) The pH of the final solution is:
a) 12.00, b) 12.70, c) 13.18, d) 12.40
Model Answer
(i) b, (ii) d
In the problems below equal volumes of the following solutions A and B are mixed:
Solution A: CH3COOH (c = 0.150 mol dm3 ),
Solution B: NaOH (c = 0.100 mol dm3 )
(i) The final solution:
a) contains a weak acid; b) contains a strong base; c) is a buffer solution; d) none of the above
(ii) The pH of the final solution is:
a) 3.17, b) 7.00, c) 5.05, d) 13.00
Model Answer
(i) c, (ii) c
In the problems below equal volumes of the following solutions A and B are mixed:
Solution A: CH3COOH (c = 0.100 mol dm3 ),
Solution B: NaOH (c = 0.100 mol dm3 )
(i) The final solution:
a) contains a weak acid; b) contains a strong base; c) is a buffer solution, d) none of the above
(ii) The pH of the final solution is:
a) 7.00, b) 13.00, c) 8.72, d) 3.02.
Model Answer
(i) c, (ii) c