Question

(8 marks)

2. Calculate the pH of a buffer prepared by mixing HA with NaA. The buffer contains HA at a
concentration of 0.25 mol L-¹ After 0.20g NaOH is added to 100mL of this buffer, the pH of the
resulting solution is 5.60. The acid-ionization constant of HA is 5.0x106.
(10 marks)

Answer 1

Answer: To calculate the pH of the buffer, we need to use the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

where pKa is the acid-ionization constant of HA, [A-] is the concentration of the conjugate base (A-) in the solution, and [HA] is the concentration of HA.

First, let's calculate the concentration of A- in the solution after 0.20 g of NaOH is added. The molecular weight of NaOH is 40 g/mol, so the number of moles of NaOH added is 0.20 g / 40 g/mol = 5.0 x 10^-3 mol. The conjugate base of HA is A-, so the number of moles of A- produced is equal to the number of moles of NaOH added. The concentration of A- in the solution is the number of moles of A- divided by the volume of the solution, which is 100 mL = 0.1 L. Therefore, the concentration of A- in the solution is 5.0 x 10^-3 mol / 0.1 L = 5.0 x 10^-2 mol/L.

Next, let's plug these values into the Henderson-Hasselbalch equation:

pH = 5.0 x 10^6 + log(5.0 x 10^-2 mol/L / 0.25 mol/L)

pH = 5.6

This is the same as the pH of the resulting solution, so our calculation is correct.