Question

Q1) Compare the observed pH values for sodium bicarbonate and sodium carbonate. In terms of Ka and Kb, explain why these differ.

Q2)

a) Why do the HCl solutions have lower pH’s?

b) Why is there a more significant change in the pH of the HCl solutions than in the two acetic acid solutions ( sodium acetate and ammonium acetate)?

Answer 1

Q1) If we consider solutions of the same concentration, sodium carbonate Na₂CO₃ has a much basic pH than sodium bicarbonate NaHCO₃ (a much basic pH also means a much higher pH, for example a pH of 10 is more basic than a pH of 8). But why does this happen? Well, the carbonate ion CO₃^2- can only act as a base (remember that bases accept H⁺ according to the Bronsted-Lowry theory) while the bicarbonate ion HCO₃^- can act as both an acid and a base (because it can accept or donate a H⁺).

These are the reactions:

>Carbonate as a base: CO₃^2- + H⁺ <=> HCO₃^- (pKb = 3.68)

>Bicarbonate an acid: HCO₃^- <=> CO₃^2- + H⁺ (pKa = 10.32)

>Bicarbonate as a base: HCO₃^- + H⁺ <=> H₂CO₃ (pKb = 7.7)

So, to know which one has a higher pH (or more basic pH) just look at the pkb values for when carbonate and bicarbonate act as bases. The higher the pKb value the weaker the base (which means that a lower pKb value will result in a stronger base and a higher pH, also, the higher the pKb the lower the Kb). So, carbonate has a higher pH because it has a lower pKb and thus, a higher Kb value.

Q2) Remember that HCl is a strong acid, meaning it dissociates completely into H⁺ and Cl^- ions. Also, remember that the pH is a measure of the H⁺ concentration in solution. Also, remember that a lower pH indicates a higher H⁺ concentrations. Thus, a strong acid like HCl will have a lower pH.

Acetic acid is a weak base, it doesn't dissociate entirely so the solution will not have as much H⁺ ions as the HCl solution, thus it will have a higher pH. When we add a base to the HCl solution the pH will change abruptly but when we add a base to a weak acid the pH will change slowly because a buffer is formed (a buffer is the combination between a weak acid and its salt or a weak base and its salt).