Solveeit Logo
Login

Question

Question: Hydrolysis constants of two salts KA and KB of weak acids HA and HB are \[{10^{ - 8}}\] and \[{10^{ ...

Hydrolysis constants of two salts KA and KB of weak acids HA and HB are 10−8{10^{ - 8}} and 10−6{10^{ - 6}}. If the dissociation constant of third acid HC is 10−2{10^{ - 2}}. The order of acidic strengths of three acids will be:
A. HA > HB > HC
B. HB > HA > HC
C. HC > HA > HB
D. HA = HB = HC

Explanation

Solution

The dissociation constant is a measure of acid strength in which an acid with higher Ka{K_a} value, will liberate greater number of hydrogen ions per mole of acid in solution and hence acid will be called stronger. It is also known as acidity constant, or acid-ionization constant. It is the equilibrium constant for a chemical reaction HA⇌A−+H+HA \rightleftharpoons {A^ - } + {H^ + } known as dissociation in the context of acid–base reactions. The chemical species HA is an acid that dissociates into A−{A^ - }. The system is said to be in equilibrium when the concentrations of its components will not change with the passing of time, because both forward and backward reactions are occurring at the same rate.
The relation between Ka{K_a}, KW{K_W} and Kh{K_h} is given by Kh=KWKa{K_h} = \dfrac{{{K_W}}}{{{K_a}}}
Apply relationship between ionic product of water and hydrolysis constant to get Ka{K_a}

Complete step by step answer:
The acid dissociation constant Ka{K_a} is the ratio of the ionic product of water KW{K_W} to the hydrolysis constant Kh{K_h}.
Dissociation of acid Ka=Ionic product of water Hydrolysis constant    {K_a} = \dfrac{{Ionic{\text{ }}product{\text{ }}of{\text{ }}water{\text{ }}}}{{Hydrolysis{\text{ }}constant}}\;\;
Ionic product of water, Kw=10−14{K_w} = {10^{ - 14}} (a constant)
For acid HA, Ka=KwKh=10−1410−8=10−6{K_a} = \dfrac{{{K_w}}}{{{K_h}}} = \dfrac{{{{10}^{ - 14}}}}{{{{10}^{ - 8}}}} = {10^{ - 6}}
For acid HB, Ka=KwKh=10−1410−6=10−8{K_a} = \dfrac{{{K_w}}}{{{K_h}}} = \dfrac{{{{10}^{ - 14}}}}{{{{10}^{ - 6}}}} = {10^{ - 8}}
For acid HC, Ka=10−2{K_a} = {10^{ - 2}}
Comparing Ka{K_a}values, it is found that Ka{K_a}of HC is greatest and Ka{K_a} of HB is least and Ka{K_a} of HA lies in between
Now, as mentioned above, the acid strength is directly proportional to Ka{K_a} values.
Therefore, the order of acid strengths is:
HC>HA>HBHC > HA > HB

That is, option C. is the correct one.

Note: Kw{K_w} and Kh{K_h} values should be used to calculate Ka{K_a} and directly find the acidic strength by using the relationship that higher the Ka{K_a} value, higher the acidic strength.