Question

The ${K_a}$ value for hydrochloric acid is $1.76 \times {10^{ - 5}},$ and the ${K_a}$ value for the benzoic acid is $6.46 \times {10^{ - 5}},$ if two solutions are made, one from each acid, with equal concentration, which will have the lower pH?

Answer 1

Hint: For any acid chemical reaction ${K_a}$ is represented as acid dissociation constant, which gives us knowledge about the nature of acid it means we are able to ensure that given acid is weak acid or strong acid.

Step by step answer: ${K_a}$ is also known as acidity constant or acid – ionization constant and For some chemical reactions ${K_a}$ is also consider as equilibrium constant which is the ratio of the concentration of products and reactants.
For example: $A + B \to C,$ then in this condition value of ${K_a}$ at equilibrium is expressed as follow:

  ${K_a} = \frac{{\left[ C \right]}}{{\left[ A \right]\left[ B \right]}}$

From the above points it is clear that:

If the value of ${K_a}$ is high it means in a chemical reaction more dissociation of acid takes place due to which high amount of product is formed and it favors the reaction, so in this condition present acid is a strong acid.

If the value of ${K_a}$ is low it means in a chemical reaction more dissociation of acid doesn’t take place due to which a high amount of product is not formed and it doesn’t favors the reaction, so in this condition present acid is a weak acid.
In the given question we have to find out the pH of the formed solution and pH of any solution is directly proportional to the concentration of ${H^ + }$ ion or hydronium ion ( ${H_3}{O^ + }$ ), because pH is calculated as:

$pH = - \log \left[ {{H^ + }} \right]$

pH of any acid is ranging between 0 to 7, and strong acids show low pH value & weak acids show high pH value. From the given data it is clear that:
As the value of ${K_a}$ for any solution is high so the concentration of ${H^ + }$ ions also high, and the solution will given lower pH because they are more acidic than those solution whose ${K_a}$ value is less.
From the given data in question we can conclude that the solution of benzoic acid shows lower pH because the value of ${K_a}$ for this acid is high.

Note: In this question you may be confused why the value of pH for strong acid is not higher than weak base, so the reason is that in a pH scale acidity decreases as we move forward from 0 to 14, and 14 shows the high basic nature.