Question

A solution is made up by combining 50 ml of a 1M acetic acid solution and 50 ml of a 1.13 M sodium acetate solution. Then, 400 ml of water is added. What is the pH of the new solution? Ka of acetic acid is 1.8x10^-5.

Answer 1

Mixing acetic acid (weak acid) and sodium acetate ( salt of conjugate base of weak acid) makes a buffer solution.

Millimoles of acetic acid = volume of acetic acid x molarity of acetic acid = 50 mL x 1 M = 50 mmol

Millimoles of sodium acetate = volume x molarity = 50 mL x 1.13 M = 56.5 mmol

pH of the buffer can be calculated by using Henderson equation.

pH = pKa + log ( [sodium acetate] / [acetic acid])

OR

pH = -log(Ka) + log ( mmol sodium acetate/mmol acetic acid)

pH = -log(1.8x10^-5 ) + log (56.5 mmol/50 mmol)

    = -(-4.74) + (0.053)

   = 4.79