Understanding pH and pOH in Chemistry

Calculating [OH−] Concentration and pOH of a Solution

A chemist is using a solution of HNO₃ that has a pH of 3.75. They need to determine the concentration of hydroxyl ions [OH⁻] for the solution and also find out the pOH of the solution.

Calculating [OH−] Concentration

Answer:

Explanation:

Given parameters:

pH of HNO₃ = 3.75

Unknown:

[OH⁻], concentration of hydroxyl ions = ?

pOH of the solution = ?

Solution

The pH or pOH scale is a convenient way of expressing the level of acidity or alkalinity of aqueous solutions.

The pH of a solution is the negative logarithm to base 10 of the hydrogen ion concentration of the solution:

pH = -log₁₀[H⁺]

pOH = -log₁₀[OH⁻]

For any aqueous solution, the sum of the pH and pOH is 14. That is;

pH + pOH = 14

Now solving for [OH⁻]:

HNO₃ + H₂O → H₃O⁺ + NO₃⁻

Since pH + pOH = 14

pOH = 14 - pH = 14 - 3.75 = 10.25

since pOH = -log₁₀[OH⁻]

10.25 = -log₁₀[OH⁻]

[OH⁻] = inverse log₁₀(-pOH)

[OH⁻] = inverse log₁₀(-10.25) = 5.62 x 10⁻¹¹ moldm⁻³