How does dilution affect the pH of a basic solution?

May 5, 2014

Dilution reduces the pH of a basic solution.

Explanation:

The pH of a basic solution is greater than 7.

As you dilute a solution, it becomes more and more like pure water. So the pH moves closer to the pH of pure water, pH 7. The pH decreases on dilution.

Example:

What is the pH of a 10⁻³ mol/L solution of NaOH? What is the pH if you dilute 100 mL of this solution to 1 L?

Solution:

pOH = -log[OH⁻] = -log(10⁻³) = 3.

pH = 14 – pOH = 14 – 3 = 11.

For dilution,

${c}_{1} {V}_{1} = {c}_{2} {V}_{2}$

c_2 = c_1 × V_1/V_2 = 10⁻³ mol/L × $\left(100 \text{ mL")/(1000" mL}\right)$ = 10⁻⁴ mol/L

pOH = -log[OH⁻] = -log(10⁻⁴) = 4.

pH = 14 – pOH = 14 – 4 = 10.

The pH has decreased from 11 to 10.

The pH of a basic solution decreases on dilution.