Question

Calculate the conjugate base to acid ratio in Solution D (10 mL of 0.2 M acetic acid and 1 mL of 0.1 M sodium hydroxide, and 9 mL distilled water). Then calculate the theoretical pH.

Initial pH: 3.47

Molarity of HCl: 0.0992        about 0.3 mL added               pH after HCl addition: 3.09

Molarity of added NaOH: 0.0997M         about 0.2 mL added           pH after NaOH addition: 3.54

Answer 1

Solution D

Ch3COOH + NaOH -----------------> CH3COONa +H2O

10x 0.2 1x0.1 0 0 initial mmoles

1.0 0 1.0 0 after reaction

1/20 0 1/20 concentrations

Thus the ratio of conjugate bse to acid = 1.0/1.0 = 1:1

The solution D is a buffer

Its pH is calculated using Hendersen equation

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

Ka of acetic acid = 1.8x10^-5 and pKa = 4.74

thus pH = 4.74 + log 1

= 4.74

II)

when HCl is added to this buffer

CH3COOH -------------------> CH3COO- + H+

20x0.1 20x0.1 0 initial mmoles

0.3x0.0992=0.02976 change

2.02976   1.970 24 0 after reaction

2.02976/20.3   1.97024/20.3    0 concentrations

Thus pH of the neww buffer

pH = 4.74 + log 1.97024 /2.02976

= 4.427

iii) after NaOH is added

CH3COOH + NaOH ----------------------> CH3COONa

20x0.1 0 20x0.1 initial mmoles

0.2 x0.0997=0.01994 change

1.98006 0 2.01994 after reaaction

1.98006/20.2 2.01994/20.2 concentrations

thus the pH of this solution is

pH = 4.74 + log 2.01994/1.98006

= 4.7486