pH Scale and Calculations - SS2 Chemistry Lesson Note

The pH scale is a logarithmic scale used to measure the acidity or alkalinity (basicity) of a solution. It quantifies the concentration of hydrogen ions (H+) present in a solution. The pH scale ranges from 0 to 14, with 7 being considered neutral. pH values below 7 indicate acidity, while pH values above 7 indicate alkalinity. The pH of a solution is calculated using the formula: pH = -log[H+], where [H+] represents the concentration of hydrogen ions in moles per litre. Inversely, the concentration of hydrogen ions can be calculated using the formula: [H+] = 10^(-pH).

The pH scale is logarithmic, meaning that each unit on the scale represents a tenfold difference in hydrogen ion concentration. For example, a solution with a pH of 4 has ten times higher hydrogen ion concentration than a solution with a pH of 5. Strong acids have a low pH value, as they ionise completely in water and release a high concentration of hydrogen ions. Examples include hydrochloric acid (pH ≈ 1) and sulfuric acid (pH ≈ 0).

Strong bases have a high pH value, as they dissociate completely in water and generate a high concentration of hydroxide ions (OH-), which indirectly affects the hydrogen ion concentration. Examples include sodium hydroxide (pH ≈ 14) and potassium hydroxide (pH ≈ 13). Pure water has a pH of 7, as it contains equal concentrations of hydrogen ions and hydroxide ions, making it neutral. Acidic solutions have a pH lower than 7, and their hydrogen ion concentration is greater than their hydroxide ion concentration.

Alkaline or basic solutions have a pH higher than 7, and their hydroxide ion concentration is greater than their hydrogen ion concentration. The pH scale allows us to compare the relative acidity or alkalinity of different solutions and assess their chemical behaviour and potential effects. The pH of a solution can be measured using pH indicators, pH metres, or by using a known concentration of an acid or base to titrate the solution. pH plays a crucial role in various fields such as chemistry, biology, environmental science, and medicine. It affects chemical reactions, enzyme activity, biological processes, and the behaviour of substances in natural and industrial systems.

Understanding the pH scale and being able to calculate pH values is essential for analysing and controlling acid-base reactions, understanding the behaviour of solutions, and interpreting experimental data in various scientific disciplines.