Understanding pH and pOH in Chemical Solutions

Expert reviewed • 22 November 2024 • 5 minute read

Introduction

pH and pOH are fundamental concepts in chemistry that help us quantify the acidity and basicity of solutions. This article explores their calculations and relationships, with a focus on HSC Chemistry requirements under the Brønsted-Lowry Theory topic.

Understanding pH and Hydrogen Ions

pH measures the concentration of hydrogen ions (H⁺) in a solution. The relationship between pH and hydrogen ion concentration is expressed through the following equation:

pH=−log⁡10[H+]\text{pH} = -\log_{10}[\text{H}^+]

Conversely, to find the hydrogen ion concentration from pH:

[H+]=10−pH[\text{H}^+] = 10^{-\text{pH}}

Understanding pOH and Hydroxide Ions

Similar to pH, pOH measures the concentration of hydroxide ions (OH⁻) in a solution:

pOH=−log⁡10[OH−]\text{pOH} = -\log_{10}[\text{OH}^-]

And to calculate hydroxide ion concentration:

[OH−]=10−pOH[\text{OH}^-] = 10^{-\text{pOH}}

The Water Self-Ionization Equilibrium

Water undergoes self-ionization according to the following equation:

2H2O(l)⇌H3O+(aq)+OH−(aq)2\text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{OH}^-(aq)

At 25°C, the ion product constant of water (K_w) is:

Kw=[H3O+][OH−]=1.0×10−14K_w = [\text{H}_3\text{O}^+][\text{OH}^-] = 1.0 \times 10^{-14}

This leads to the important relationship:

pH+pOH=14 (at 25°C)\text{pH} + \text{pOH} = 14 \text{ (at 25°C)}

Significant Figures in pH Calculations

When working with pH and pOH calculations, follow these rules:

  • The number of decimal places in pH/pOH equals the number of significant figures in the ion concentration
  • When converting from pH to concentration, match significant figures to the decimal places in pH

Practice Question 1

Calculate the pH of a solution with [H⁺] = 0.0010 mol/L

pH=−log⁡10[0.0010]=3.00\text{pH} = -\log_{10}[0.0010] = 3.00

Note: The answer is given to 2 decimal places because the concentration has 2 significant figures.

Practice Question 2

Find [H⁺] in a solution with pH = 4.35

[H+]=10−4.35=4.5×10−5 mol/L[\text{H}^+] = 10^{-4.35} = 4.5 \times 10^{-5} \text{ mol/L}

Note: The answer is given to 2 significant figures to match the 2 decimal places in pH.

Key Points to Remember

  • pH and pOH are logarithmic scales
  • A change of 1 unit represents a 10-fold change in concentration
  • Lower pH means higher acidity (higher [H⁺])
  • The pH + pOH = 14 relationship only holds at 25°C

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