Question

Consider the following data on some weak acids and weak bases:
acid base
name formula Ka name formula Kb
hydrocyanic acid HCN 4.9 x 10^-10 hydroxylamine HONH2 1.1 x 10^-1
hypochlorous acid HCIA 3.0 x 10^-18 ethylamine C2H5NH2 6.4 x 10^-4
Use this data to rank the following solutions in order of increasing pH. In other words, select a '' next to the solution that will have the lowest pH, a '' next to the solution that will have the next lowest pH, and so on.
a. 0.1 M HONH3Br
b. 0.1 M NaNO3
c. 0.1 M C2H5NH3Cl
d. 0.1 M NaF

Answer 1

Answer:

a < c < b < d

Explanation:

The weak acid with the lowest pKa will be the most acidic. In the other way, the conjugate base which the acid is weakest will be strongest.  

The weak base with the lowest pKb will be the most basic. And the conjugate base of the weakest base will be a strongest acid.

Using the values:

Ka HCN = 4.9x10⁻¹⁰

Kb HONH2 = 1.1x10⁻⁸

Ka HClA 3.0x10⁻⁸

Kb C2H5NH2 = 6.4x10⁻⁴

The NaNO3 is the conjugate base of a strong acid as HNO3. That means its solutions are almost neutral

The HONH3Br is the conjugate acid of a weak base. Its solutions will be acidics

C2H5NH3Cl is the conjugate acid of a weak base as ethylamine, its solutions will be acidic.

As ethylamine has a lower pkb than hydroxylamine, the conjugate acid of hydroxylamine will be more acidic.

NaF is the conjugate base of a weak acid as HF. Its solutions will be basics

The order in increasing pH is:

HONH3Br < C2H5NH3Cl < NaNO3 < NaF

a < c < b < d