Question

Addition of 0.1 M HCI 9.00 +H2O -CH3COOH CH3COO- CH3COOH/CH3COO- Plot Area 200 Drops data HCI NaOH +

Describe, in general terms, the similarities and differences between each of the four curves on your data plot for the addition of HCl.

Answer 1

Solution:

Similarities: There is pH drop in all the four curves due to increase in the H+ concentration as more drops of 0.1M HCl is added.

Differences:

1. H2O curve: Initial pH about 6.0. As the HCl is added, the pH starts dropping towards more acidic. This is because gradual increase in H3O+ ions. The equilibrium reaction is, H2O + H+   $\rightleftharpoons$ H3O+ and shift to the left side.
2. CH3COOH curve: Initial pH is about 3.5. This is because CH3COOH is a weak monoprotic acid and merely dissociates to its conjugate base, CH3COO− ions. The equilibrium reaction is, CH3COOH $\rightleftharpoons$ H3O+ + CH3COO− . Addition of HCl increases the H3O+ concentration that shifts the reaction to the left. Hence the pH is about 2.5 - 3 till the H3O+ equals the  CH3COO−. Further addition of HCl increases the H3O+ concentration. Hence the pH drops.
3. CH3COO− curve: CH3COO− ion is the conjugate base of weak acid, CH3COOH. Initial pH is about 8.2 because this conjugate base is strong base. When HCl drops are added to the CH3COO− ions are converted to CH3COOH. Further addition of HCl increases the H3O+ concentration. Hence the pH drops.
4. CH3COOH / CH3COO− curve: This is buffer. Initial pH is about 4.5. As the HCl addition, the CH3COO− ions are converted to CH3COOH. During this time, the solution pH does not change much due to buffering action. Further addition of HCl increases the H3O+ concentration. Hence the pH drops.