Arrange the solutions in order of increasing acidity. Given Ka & Kb values

Question

Arrange the solutions in order of increasing acidity.

Ka (NH₄⁺) = 5.6 x 10-10

Kb (ClO₂^-) = 9.1 x 10-13

NaCl, NH₄Cl, NaHCO₃, NH4ClO₂, NaOH

Where to even begin?

Answer 1

There are several ways to approach this problem. Since you state "where to even begin", perhaps the best way to explain this, is to begin at the beginning. Of course, even though not stated in the problem, we will have to assume that all are present at the same concentration, otherwise, the comparison is not justified.

Let us try to get an estimate of the pH of each of the compounds listed.

NaCl, NH₄Cl, NaHCO₃, NH₄ClO₂, NaOH

Recall that a salt made from ...

a strong acid + strong base will have pH = 7 (neutral)

a strong acid + weak base will have pH < 7 (acidic)

a weak acid + strong base will have pH > 7 (basic)

a weak acid + weak base will have a pH dependent on the relative values of the respective Ka & Kb values

-The easy one is NaOH, as it is a strong base, so it will have high pH. The highest of all of them.

-NaCl is the salt of a strong acid (HCl) and a strong base (NaOH, so it will have a pH = 7.

-NH4Cl is the salt of a strong acid (HCl) and a weak base (NH₄OH), so it will have a pH <7.

-NaHCO₃ is the salt of a weak acid (H₂CO₃) and a strong base (NaOH), so it will be slightly basic.

-NH₄ClO₂ is the salt of a weak acid (HClO₂) and a weak base (NH₄OH). When you have this situation, we have to compare Ka to Kb. In this case Ka for NH₄⁺ (5.6x10^-10) is greater than Kb for ClO₂^- (9.1x10^-13), so this solution will be slightly acidic.

RANKING THEM FROM LEAST ACIDIC TO MOST ACIDIC...

NaOH < NaHCO₃ < NaCl, NH₄ClO₂ < NH₄Cl

least acidic.........................................most acidic