Based on the following titration curve, what is the Ka for this weak acid?

Question

BlackTom AI · Accepted Answer

To determine Ka from a titration curve of a weak acid with strong base, follow these steps:

- Identify the equivalence point volume from the curve where the pH rises most steeply. In this plot, the sharp vertical rise occurs around 7.5 to 8 mL of NaOH added, indicating the equivalence point is near 7.8–8.0 mL.
- Use the half-equivalence concept: at half of the equivalence volume, the concentrations of the weak acid (HA) and its conjugate base (A−) are equal, [HA] = [A−], and the pH equals pKa. Therefore, read the pH at exactly half the equivalence volume.
- Calculate half of the equivalence volume: if equivalence is about 8.0 mL, half is ~4.0 mL. Look at the graph around 4.0 mL; the pH there is roughly 4.7 (slightly below 4.8). This pH value corresponds to pKa.
- Convert pKa to Ka using Ka = 10^(−pKa). If pKa ≈ 4.7, then Ka ≈ 10^(−4.7) ≈ 2 × 10^−5. A value around 2.5 × 10^−5 is consistent with pKa ≈ 4.6–4.7.

Common pitfalls to avoid:
- Misreading the equivalence point: misplacing the steep rise shifts half-equivalence readings and hence pKa estimation.
- Assuming pH at the equivalence point equals pKw or pH 7 for all weak acids; for weak acid–strong base titrations, equivalence pH is typically >7 due to the basic conjugate base.
- Using the initial pH or final pH to deduce Ka; those values reflect only the starting amount of HA or the fully neutralized solution, not the acid’s dissociation constant.

If you have access to precise numbers for the equivalence volume (Ve) and the pH exactly at Ve/2, you can compute pKa as the pH at Ve/2, and then Ka = 10^(−pKa). Based on the graph, Ve ≈ 8 mL and pH at Ve/2 ≈ 4.7, giving Ka ≈ 2–3 × 10^−5, which is close to 2.51 × 10^−5.

Based on the following titration curve, what is the Ka for this weak acid? Single choice

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