Calculate pH of Buffer Solution and Its Variations : A Lesson in Chemistry
How can we calculate the pH of a buffer solution composed of benzoic acid and sodium benzoate?
a) The pH of the buffer solution is approximately 4.48.
What happens to the pH after 0.020 mol of HCl have been added to the buffer solution?
b) After adding 0.020 mol of HCl, the pH of the solution is approximately 4.31.
And what about the pH after adding 0.020 mol of NaOH to the buffer solution?
c) After adding 0.020 mol of NaOH, the pH of the solution is approximately 4.72.
a) To calculate the pH of the buffer solution composed of benzoic acid and sodium benzoate, we need to consider the dissociation of benzoic acid. The dissociation constant, Ka, of benzoic acid is given as 6.3×10^-5. Benzoic acid (C6H5COOH) dissociates into its conjugate base, benzoate ion (C6H5COO-), and a hydrogen ion (H+):
b) After adding 0.020 mol of HCl, the H+ ions react with the benzoate ions to form more benzoic acid. The amount of benzoate ions decreases, while the amount of benzoic acid increases.
c) After adding 0.020 mol of NaOH, the OH- ions react with the H+ ions to form water. The amount of H+ ions decreases, while the amount of water increases.
a) To calculate the pH of the buffer solution, we first need to determine the initial concentrations of benzoic acid and benzoate ions. Given the concentrations of 0.12 mol benzoic acid and 0.20 mol sodium benzoate, we can use the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
where pKa = -log(Ka) and [A-] and [HA] are the concentrations of benzoate ions and benzoic acid, respectively.
Calculating the pH, we get approximately 4.48, indicating the slightly acidic nature of the buffer solution.
b) By adding 0.020 mol of HCl, the equilibrium shifts as H+ ions combine with benzoate ions to form more benzoic acid. This results in a decrease in benzoate ions and an increase in benzoic acid concentration, altering the pH to around 4.31, still within the acidic range.
c) Conversely, introducing 0.020 mol of NaOH leads to the reaction of OH- ions with H+ ions, forming water. The decrease in H+ ions and increase in water content causes the pH to rise to approximately 4.72, reflecting a more alkaline environment compared to the initial pH.