Question

Which of the following will occur if a 1.0M solution of a weak acid is diluted to 0.01M at constant temperature:-
(A) Percentage ionization will increase
(B) $[{H^ + }]$ will decrease to 0.01M
(C) ${K_c}$ will increase
(D) pH will decrease by 2 units.

Answer 1

Weak acid dissociates partially which leads to equilibrium. When the reaction condition is changed then the equilibrium is disturbed which changes the result.

Complete step by step answer:
A weak acid is the acid which dissociates partially in water. The dissociation reaction is given as follows:
$HA \rightleftharpoons {H^ + } + {A^ - }$
Since the reaction is in equilibrium, then according to the law of chemical equilibrium,
${K_c} = \dfrac{{[{H^ + }][{A^ - }]}}{{[HA]}}$
$[{H^ + }]$ = concentration of hydrogen ions in weak acid in
$[HA]$ = concentration of weak acid in
${K_c}$ = acid dissociation constant for weak acid.
Here ${K_a}$ is temperature dependent. As per the question since the dilution of the weak acid is taking place at constant temperature thus the value of ${K_a}$ remains constant. In this way we can eliminate option (C).

In case of dilution of weak acid: When we add water to the solution, the volume of the solution is increasing. In this case the equilibrium is disturbed. So according to Le Chatlier’s principle, any change in the reaction medium will cause disturbance in equilibrium and it will show the tendency to restore the equilibrium. So now in this case, as volume is increased the equilibrium will shift towards the direction where more moles of product are formed. This will result in an increase in the number of $[{H^ + }]$ ions in the solution. So we can eliminate the option (B).

The pH of the solution is the measure of the acidity and basicity of the substance. Acids are solutions with pH value less than 7. It is equal to the negative of the base 10 logarithm of hydrogen ion $[{H^ + }]$.
In case of weak acids, it does not dissociate completely in water. Thus it becomes difficult to calculate the pH value. But on dilution we have seen that there is an increase in the number of $[{H^ + }]$ ions. From the equation we can observe that the pH is directly proportional to the concentration of $[{H^ + }]$, thus the pH will also increase. We can now eliminate the option (D).

The dissociation of weak acid is also expressed in terms of degree of dissociation. The degree of dissociation is given by following formula:
$\alpha = \sqrt {\dfrac{{{K_c}}}{c}}$
As we have observed that on dilution the number of ions increases which indicate that more dissociation has taken place. Here we can compare dissociation with ionization as weak acid HA is getting ionized to ${H^ + }$ and ${A^ - }$.
So, the correct answer is “Option A”.

Note: Ionization of the compound is the process of conversion of neutral compounds into charged ions when exposed to solution. Here in case of weak acid, acid is the neutral compound which dissociates to hydrogen ions and corresponding conjugate base. These are charged species. Hence we can consider dissociation as part of ionization.