In: Chemistry
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
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