Question

A student wishes to prepare a buffer solution by adding 0.20 moles of acetic acid (Ka = 1.8 x 10^-5) and 0.20 moles of sodium acetate in 1 liter of solution. The student adds 0.05 moles of hydrochloric acid (HCl) to the solution. Calculate the effect of adding the buffer on the equilibrium position of the acid-base reaction and determine the new pH of the solution.

Answer 1

To calculate the effect of adding the buffer on the equilibrium position of the acid-base reaction and determine the new pH of the solution, we can use the Henderson-Hasselbalch equation:pH = pKa + log [A-]/[HA] where pH is the pH of the solution, pKa is the negative logarithm of the acid dissociation constant  Ka , [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.First, we need to find the pKa of acetic acid:pKa = -log Ka  = -log 1.8 x 10^-5  = 4.74Next, we need to determine the new concentrations of acetic acid and acetate ion after adding 0.05 moles of HCl. The HCl will react with the acetate ion  CH3COO-  to form acetic acid  CH3COOH  and chloride ion  Cl- :CH3COO- + HCl -> CH3COOH + Cl-Since 0.05 moles of HCl are added, 0.05 moles of CH3COO- will react to form 0.05 moles of CH3COOH. The new concentrations are:[CH3COO-] = 0.20 moles - 0.05 moles = 0.15 moles[CH3COOH] = 0.20 moles + 0.05 moles = 0.25 molesNow we can plug these values into the Henderson-Hasselbalch equation:pH = 4.74 + log 0.15/0.25  = 4.74 - 0.40 = 4.34The new pH of the solution after adding 0.05 moles of HCl is 4.34.