Question

A 2.5 M solution of a monoprotic acid has a pH of 2.50 at 25°C. What is the equilibrium constant for this acid? Is it a strong or weak acid?

Answer 1

4.0 × 10⁻⁶

Weak acid

Step-by-step explanation:

Step 1: Calculate [H⁺]

We know that the pH is 2.50. We will calculate [H⁺] using the following expression.

pH = -log [H⁺]

[H⁺] = antilog -pH = antilog -2.50 = 3.16 × 10⁻³ M

Step 2: Calculate the acid dissociation constant (Ka)

We will use the following expression.

`Ka = ([H^(+)]^(2) )/(Ca)`

where,

Ca is the concentration of the acid (2.5 M)

`Ka = ([H^(+)]^(2) )/(Ca) = ((3.16 * 10^(-3) )^(2) )/(2.5)= 4.0 * 10^(-6)`

Since Ka << 1, the acid is weak.