Understanding pH and pOH in Chemical Solutions

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:

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

Conversely, to find the hydrogen ion concentration from 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:

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

And to calculate hydroxide ion concentration:

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

The Water Self-Ionization Equilibrium

Water undergoes self-ionization according to the following equation:

$2\text{H}_2\text{O}(l) \rightleftharpoons \text{H}_3\text{O}^+(aq) + \text{OH}^-(aq)$

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

$K_w = [\text{H}_3\text{O}^+][\text{OH}^-] = 1.0 \times 10^{-14}$

This leads to the important relationship:

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

Significant Figures in pH Calculations

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