This set of Biochemistry Questions and Answers for Freshers focuses on “Buffering against pH changes in Biological Systems”.
1. Which can act as buffer?
a) NH4 Cl + HCl
b) CH3 COOH + H2 CO3
c) 40ml of 0.1M NaCN + 20ml of 0.1M HCN
d) NaCl + NaOH
Explanation: It is a mixture of weak acid and its salt with strong base.
2. Calculate the pH of a mixture of 0.10M acetic acid and 0.20M sodium acetate. The pKa of acetic acid is 4.76.
Explanation: pH = pKa + log [acetate]/[acetic acid] = 4.76 + log (0.2/0.1)
= 4.76 + 0.30
3. Calculate the pKa of lactic acid, given that when the concentration of lactic acid is 0.010M and the concentration of lactate is 0.087M, the pH is 4.80.
Explanation: pH = pKa + log [lactate]/[lactic acid] pKa = pH – log [lactate]/[lactic acid] = 4.80 – log (0.087/0.010) = 4.80 – log 8.7
4. Calculate the ratio of the concentrations of acetate and acetic acid required in a buffer system of pH 5.30.
Explanation: pH = pKa + log [acetate]/[acetic acid] log [acetate]/[acetic acid] = pH – pKa
= 5.30 – 4.76 = 0.54
[acetate]/ [acetic acid] = antilog 0.54 = 3.5.
5. Assertion A: pH of a buffer solution solution does not change on dilution.
Reason R: On dilution the ration of concentration of salt and acid (or base) remains unchanged.
a) A and R are true, R is the correct explanation of A
b) A and R are true, R is not correct explanation of A
c) A is true but R is false
d) A is false but R is true
Explanation: On dilution pH of buffer solution remains unchanged because the ratio of concentration of salt and acid (or base) remains unchanged.
6. Which of the following mixture in aqueous solution of equimolar concentration acts as a buffer solution?
a) HNO3 + NaOH
b) H2 SO4 + KOH
c) NH4 OH(excess) + HCl
d) CH3 COOH + NaOH(excess)
Explanation: NH4 OH(excess) + HCl → NH4 Cl + H2 O
So the mixture contains NH4 OH + NH4 Cl.
7. 1M NaCl and 1M HCl are present in an aqueous solution. The solution is
a) Not a buffer solution with pH < 7
b) Not a buffer solution with pH > 7
c) A buffer solution with pH < 7
d) A buffer solution with pH > 7
Explanation: It is a neutral solution and its pH = 7.
8. For an acid buffer solution the pH is 3. The pH can be increased by
a) Increasing the concentration of salt
b) Increasing the concentration of acid
c) Decreasing the concentration of salt
d) Independent of concentration of acid & salt
Explanation: An acid buffer is a mixture of weak acid and its salt with strong base, its pH increases by increasing the concentration of salt.
9. The buffer capacity is equal to
a) Δn / ΔpH
b) pH / Δn
c) ± 1pKa
d) ± 2pKa
Explanation: Buffer capacity = number of moles of acid or base added to 1 liter of buffer/change in pH.
10. Buffer capacity is maximum when
a) One mole of NH4 Cl is added to two moles of NH4 OH
b) One mole of NH4 Cl is added to one moles of NH4 OH
c) One mole of NH4 Cl is added to one mole of NaOH
d) One mole of NaCl is added to one mole of NaOH
Explanation: Buffer has more capacity when pH = pKa.
Sanfoundry Global Education & Learning Series – Biochemistry.
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