Question

9. A buffer is made by mixing 85 mL of 0.13 M lactic acid (HC3H5O3) with 95 mL of 0.15 M sodium lactate.

a. Calculate the pH of the buffer.

b. Calculate the pH after 3.0 mL of a 0.20 M HCl solution is added to the buffer.

c. Calculate the pH after 3.0 mL of a 0.20 M NaOH solution is added to the buffer INSTEAD of the HCl solution.

d. Calculate the pH of the buffer solution if 3.0 mL of a 0.20 M NaOH solution is added after 3.0 mL of a 0.20 M HCl solution is added to the buffer.

ALL PARTS A-D and show all work PLEASE!!

Answer 1

pKa of lactic acid = 3.86

a. Calculate the pH of the buffer.

millimoles of lactic acid = 85 x 0.13 = 11.05

millimoles of sodium lactate = 95 x 0.15 = 14.25

pH = pKa + log [sodium lactate / lactic acid]

pH = 3.86 + log (14.25 / 11.05)

pH = 3.97

b. Calculate the pH after 3.0 mL of a 0.20 M HCl solution is added to the buffer.

millimoles of HCl = 3 x 0.2 = 0.6 = C

pH = pKa + log [sodium lactate - C / lactic acid + C]

pH = 3.86 + log (14.25 -0.6 / 11.05 +0.6)

pH = 3.93

c. Calculate the pH after 3.0 mL of a 0.20 M NaOH solution is added to the buffer INSTEAD of the HCl solution.

pH = pKa + log [sodium lactate + C / lactic acid - C]

pH = 3.86 + log (14.25 + 0.6 / 11.05 -0.6)

pH = 4.01

d. Calculate the pH of the buffer solution if 3.0 mL of a 0.20 M NaOH solution is added after 3.0 mL of a 0.20 M HCl solution is added to the buffer.

in this case there is no change in pH because acid and base are neutralised each other and original buffer pH retains

pH = 3.86 + log (14.25 -0.6 +0.6 / 11.05 +0.6 -.06)

pH = 3.97