Question

A student wants to know how the equilibrium position of a weak acid such as acetic acid would be affected if they add more acetate ion. If the initial concentration of acetic acid is 0.1 M and initial concentration of acetate ion is 0.05 M. What will be the new concentration of acetate ion necessary to increase the pH of the solution by 0.5 units?

Answer 1

To solve this problem, we can use the Henderson-Hasselbalch equation, which relates the pH, pKa, and the ratio of the concentrations of the conjugate base  A-  and the weak acid  HA :pH = pKa + log  [A-]/[HA] The pKa of acetic acid is 4.74. First, let's find the initial pH of the solution:Initial pH = 4.74 + log  0.05/0.1 Initial pH = 4.74 - 0.301Initial pH  4.44Now, we want to increase the pH by 0.5 units, so the new pH will be:New pH = 4.44 + 0.5New pH = 4.94Now, we can use the Henderson-Hasselbalch equation again to find the new concentration of acetate ion  [A'-] :4.94 = 4.74 + log  [A'-]/0.1 Rearrange the equation to solve for [A'-]:0.2 = log  [A'-]/0.1 10^0.2 = [A'-]/0.11.585 = [A'-]/0.1Now, solve for [A'-]:[A'-] = 1.585 * 0.1[A'-]  0.1585 MSo, the new concentration of acetate ion necessary to increase the pH of the solution by 0.5 units is approximately 0.1585 M.