Question

Calculate the pH of a buffer that is 0.13 M in lactic acid and 0.11 M in sodium lactate. Express your answer using two decimal places. Calculate the pH of a buffer formed by mixing 85 mL of 0.12 M lactic acid with 95 mL of 0.16 M sodium lactate. Express your answer using two decimal places.

Answer 1

pKa of lactic acid 3.86

i)The solution has 0.13M acid nd 0.11 M lactate.

Thus [acid] = 0.13M and [conjugate base] = 0.11M

The pH of buffer is given by Hendersen equation

pH = pKa + log [conjugate base]/[acid]

= 3.86 + log (0.11/0.13)

= 3.7874

ii)

The buffer is formed by mixing 85mL of 0.12M acid and 95 mL of0.16M lactate .

Total volume of solution = 85 +95=180mL

[acid ] in solution = [85x0.12/180 ] M

[conjugate base ] = [95x0.16/180] M

Thus pH of solution = 3.86 + log (95x0.16)/(85x0.12)

= 4.033