Question

Enough of a monoprotic acid is dissolved in water to produce a 0.0192 M solution. The pH of the resulting solution is 2.53. Calculate the Ka for the acid.

Answer 1

Concepts and reason

The concept used to solve this problem is based on acid dissociation constant.

Dissociation constant of an acid is defined as the extent to which an acid dissociates in the solution to form its respective ions.

Fundamentals

Weak acids are those acids which partially dissociate in their aqueous solution. This means that the weak acids do not donate all of their hydrogen ions in the solution. Weak acids get dissociated to very small extent.

The relation between pH and hydrogen ion concentration is given as follows:

…… (1)

Consider the dissociation of acid ,

The expression for acid dissociation constant is written as follows:

…… (2)

As the concentration of water remains nearly constant in the reaction, its concentration term is not included in the equilibrium constant expression.

The of the solution is .

Rearrange the formula given in equation (1) to determine the concentration of hydrogen ions.

Substitute for pH in the above equation.

The ICE table for the dissociation of monoprotic weak acid is given as follows:

Hence, the concentration of and ions is equal to the hydrogen ion concentration of the solution calculated from the pH of the solution, which is .

The concentration of acid at equilibrium is calculated by the equation as follows:

Substitute for in the above equation.

Substitute for , for , and for in equation (2) to calculate dissociation constant of acid.

Ans:

The value of dissociation constant of acid is .