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
What is [OH−] for the solution? 5.62×10⁻¹¹ M What is the pOH of the solution? 10.25