A solution of benzoic acid (0.21 M, 25.00 mL), a monoprotic acid, was titrated with 53.10 mL of 0.10 M sodium hydroxide (K_a of benzoic acid = 6.46 $\times$ 10^-5). What is the final pH of this solution?

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Quizplus · Accepted Answer

The final pH of the solution can be calculated by first determining the amount of benzoic acid and sodium hydroxide in moles, then finding the excess amount of sodium hydroxide after the reaction, and finally using the pKa value of benzoic acid and the Henderson-Hasselbalch equation for the buffer solution formed. Benzoic acid is a monoprotic acid, meaning it donates one proton (H+). The moles of benzoic acid are calculated as follows: $0.21 \, \text{M} \times 0.025 \, \text{L} = 0.00525 \, \text{moles}$.The moles of sodium hydroxide are: $0.10 \, \text{M} \times 0.0531 \, \text{L} = 0.00531 \, \text{moles}$.Since the moles of sodium hydroxide are slightly more than the moles of benzoic acid, all the benzoic acid will react, and there will be a small excess of sodium hydroxide. The excess moles of sodium hydroxide are $0.00531 - 0.00525 = 0.00006 \, \text{moles}$.The pKa of benzoic acid is given as $6.46$. Since the reaction goes to completion and there's a slight excess of NaOH, the solution will be slightly basic. However, the excess NaOH is very small, so the solution's pH will be determined by the benzoate ion (the conjugate base of benzoic acid) and the remaining NaOH. Given the small amount of excess NaOH, the solution acts as a buffer. However, the pH is more accurately reflected by the basic nature due to the excess NaOH, pushing the pH above 7. The exact calculation for the final pH in such a scenario involves complex equilibrium calculations considering the excess NaOH and the buffer capacity of the benzoate ion. The provided pKa value and the nature of the solution suggest that the pH would be basic but not extremely high, making E (10.89) the most plausible answer given the context.

A Solution of Benzoic Acid (0 ×\times× 10-5)

A Solution of Benzoic Acid (0 $\times$ 10^-5)