The pH of water can be calculated using the following steps:
- Understand the concept of pH: pH is a measure of the acidity or alkalinity of a solution. It is defined as the negative logarithm of the hydrogen ion concentration ([H+]) in moles per liter.
- Recognize the auto-ionization of water: Water molecules can self-ionize, meaning they can donate a proton (H+) to another water molecule, forming a hydronium ion (H3O+) and a hydroxide ion (OH-). This equilibrium is represented by the following equation:
2H2O(l) ⇌ H3O+(aq) + OH-(aq) - Know the ion product of water (Kw): The product of the concentrations of H3O+ and OH- ions in pure water is constant at a given temperature and is called the ion product of water (Kw). At 25°C, Kw is 1.0 x 10^-14.
- Calculate the hydrogen ion concentration ([H+]): Since pure water is neutral, the concentrations of H3O+ and OH- are equal. Therefore, [H+] = √Kw = √(1.0 x 10^-14) = 1.0 x 10^-7 M.
- Calculate the pH: Using the pH formula, pH = -log[H+], we get pH = -log(1.0 x 10^-7) = 7.
Therefore, the pH of pure water at 25°C is 7.
Important Note: The pH of water can change depending on the temperature. At higher temperatures, Kw increases, and the pH decreases.
Examples:
- pH of water at 10°C: Kw = 0.29 x 10^-14, [H+] = √0.29 x 10^-14 = 5.4 x 10^-8 M, pH = -log(5.4 x 10^-8) = 7.27
- pH of water at 50°C: Kw = 5.5 x 10^-14, [H+] = √5.5 x 10^-14 = 2.3 x 10^-7 M, pH = -log(2.3 x 10^-7) = 6.64
