Calculation of pH after Titration of Weak Acid: A compound has a p K a of 7.4. T

Question

Calculation of pH after Titration of Weak Acid: A compound has a p K a of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. What is the pH of the resulting solution? Calculation of pH after Titration of Weak Acid: A compound has a p K a of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. What is the pH of the resulting solution? Calculation of pH after Titration of Weak Acid: A compound has a p K a of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. What is the pH of the resulting solution?

Explanation / Answer

Using the Henderson-Hasselbalch equation on the solution before HCl addition:
pH = pKa + log([A-]/[HA])
8.0 = 7.4 + log([A-]/[HA]);
[A-]/[HA] = 4.0. (equation 1)
Also, 0.1 L * 1.0 mol/L = 0.1 moles total of the compound. Therefore,
[A-] + [HA] = 0.1 (equation 2)
Solving the simultaneous equations 1 and 2 gives:
A- = 0.08 moles
AH = 0.02 moles
Adding strong acid reduces A- and increases AH by the same amount.
(0.03 L) (1 mol/L) = 0.03 moles HCl are added, so
A- = 0.08 - 0.03 = 0.05 moles
AH = 0.02 + 0.03 = 0.05 moles
Therefore, after HCl addition, [A-]/[HA] = 0.05 / 0.05 = 1.0
Resubstituting into the Henderson-Hasselbalch equation:
pH = 7.4 + log(1.0) = 7.4, the final pH.

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.