If the pKa of HCOOH is 3.74 and the pH of an HCOOH/NaHCOO solution is 3.11, which of the following is TRUE? A) [HCOOH] < [NaHCOO] B) [HCOOH] = [NaHCOO] C) [HCOOH] > [NaHCOO] D) It is not possible to make a buffer of this pH from HCOOH and HCOO. 6. Which of the following is TRUE? A) The equivalence point is where the amount of acid equals the amount of base during any acid-base titration. B) At the equivalence point, the pH is always 7. C) An indicator is not pH sensitive. D) A titration curve is a plot of pH vs. the [base]/[acid] ratio. E) None of the above are true.
The Correct Answer and Explanation is :
Question 5:
Given that the pKa of HCOOH is 3.74 and the pH of an HCOOH/NaHCOO solution is 3.11, we can determine the relationship between the concentrations of HCOOH and NaHCOO using the Henderson-Hasselbalch equation:
[ \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) ]
Where:
- pH = 3.11
- pKa = 3.74
- [A⁻] = [NaHCOO]
- [HA] = [HCOOH]
Substituting the known values:
[ 3.11 = 3.74 + \log \left( \frac{[\text{NaHCOO}]}{[\text{HCOOH}]} \right) ]
Rearranging to solve for the ratio of concentrations:
[ \log \left( \frac{[\text{NaHCOO}]}{[\text{HCOOH}]} \right) = 3.11 – 3.74 = -0.63 ]
Taking the antilog of both sides:
[ \frac{[\text{NaHCOO}]}{[\text{HCOOH}]} = 10^{-0.63} \approx 0.235 ]
This indicates that the concentration of NaHCOO is approximately 23.5% that of HCOOH. Therefore, [HCOOH] > [NaHCOO]. The correct answer is C) [HCOOH] > [NaHCOO].
Question 6:
Let’s evaluate each statement:
- A) The equivalence point is where the amount of acid equals the amount of base during any acid-base titration. This statement is false. The equivalence point is reached when the amount of acid equals the amount of base in the solution, but this is not true for all acid-base titrations. For example, in titrations involving weak acids or weak bases, the equivalence point may occur at a pH different from 7.
- B) At the equivalence point, the pH is always 7. This statement is false. The pH at the equivalence point depends on the strengths of the acid and base involved. For strong acid-strong base titrations, the pH at the equivalence point is 7. However, for weak acid-strong base or strong acid-weak base titrations, the pH at the equivalence point will be greater than or less than 7, respectively.
- C) An indicator is not pH sensitive. This statement is false. Indicators are specifically chosen for their pH sensitivity, as they change color at certain pH values, allowing the determination of the endpoint in titrations.
- D) A titration curve is a plot of pH vs. the [base]/[acid] ratio. This statement is false. A titration curve plots pH against the volume of titrant added, not the ratio of base to acid concentrations.
- E) None of the above are true. This statement is true. All the previous statements are false.
Therefore, the correct answer is E) None of the above are true.
For a visual explanation of the equivalence point in acid-base titrations, you might find the following video helpful: