Question

A 0.2 molar solution of formic acid is 3.2 % ionized, its ionization constant is:
a.) 9.6 $\times {{10}^{-3}}$
b.) 2.1 $\times {{10}^{-4}}$
c.) 1.25 $\times {{10}^{-6}}$
d.) 2.1 $\times {{10}^{-8}}$

Answer 1

To solve this type of question, we should first understand the basic terms which are used in this question i.e. molar solution and ionization constant. After understanding these basic terms, we can use the formulae which related the two and can solve this question.

Complete Solution :
- Molar solution is also known as molarity. Molarity is one of the standard expressions of solution concentration. The molar solution used the gram molecular weight of a solute in calculating molar concentration present in per liter of the solution.
- The ionization constant is defined as the quantitative measurement of the strength of the acid which is present in the solution.
Given in the question, a 0.2 molar solution of formic acid is 3.2% ionized. The reaction involved in the given question is:
$HCOOH+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}+COO{{H}^{-}}$

C	C	0	0
C - $C\alpha$	C-$C\alpha$	$C\alpha$	$C\alpha$

We know that the ionization constant of acid is represented as
${{K}_{a}}=\dfrac{{{C}^{2}}{{\alpha }^{2}}}{C(1-\alpha )}$
given the concentration is 0.2M
then the value of alpha will be $\alpha = 3.2$
then the value of ionization constant of acid will be:

& {{K}_{a}}=\dfrac{0.2\times {{(0.032)}^{2}}}{(1-0.032)} \\\ & {{K}_{a}}=2.1\times {{10}^{-4}} \\\ \end{aligned}$$ **So, the correct answer is “Option D”.** **Note:** We know that acids and bases can either be strong and weak. So there are different expressions of ionization constant depending on the value of degree of dissociation of acids or bases. A strong acid has the value of degree of dissociation equal to one and a weak acid has the value of degree of dissociation always less than 1 because the weak acid is partially soluble.