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
Answer: C
Clarification: 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.
A. 5.1
B. 4.1
C. 6.1
D. 7.1
Answer: A
Clarification: pH = pKa + log [acetate]/[acetic acid]
= 4.76 + log (0.2/0.1)
= 4.76 + 0.30
= 5.1.
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.
A. 4.0
B. 3.9
C. 3.3
D. 4.1
Answer: B
Clarification: pH = pKa + log [lactate]/[lactic acid]
pKa = pH – log [lactate]/[lactic acid]
= 4.80 – log (0.087/0.010) = 4.80 – log 8.7
= 3.9.
4. Calculate the ratio of the concentrations of acetate and acetic acid required in a buffer system of pH 5.30.
A. 3.2
B. 3.3
C. 3.4
D. 3.5
Answer: D
Clarification: 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
Answer: A
Clarification: 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)
Answer: C
Clarification: NH4 OH(excess) + HCl → NH4 Cl + H2O
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
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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 NH4Cl is added to two moles of NH4OH
B. One mole of NH4Cl is added to one moles of NH4OH
C. One mole of NH4Cl is added to one mole of NaOH
D. One mole of NaCl is added to one mole of NaOH
Answer: B
Clarification: Buffer has more capacity when pH = pKa.
of Biochemistry for Freshers, .