A sample of water has hardness expressed as 81 ppm Ca²⁺(aq).The sample is passed through an ion exchange column and the Ca²⁺(aq)ions are replaced by H⁺(aq)ions.What is the pH of the water after it has been treated? (1 ppm = 1 g solute/10⁶ g solution)

Question

Quizplus · Accepted Answer

The hardness of water is expressed in terms of the concentration of Ca2+ ions. 1 ppm Ca2+ (aq) means that there is 1 g of Ca2+ ions in 106 g of water. The molar mass of Ca2+ is 40.08 g/mol, so 1 ppm Ca2+ (aq) is equivalent to 1/40.08 = 0.025 mol/L Ca2+ (aq).When the water is passed through an ion exchange column, the Ca2+ (aq) ions are replaced by H+ (aq) ions. This means that the concentration of H+ (aq) ions in the water will increase. The pH of the water is a measure of the concentration of H+ (aq) ions, so the pH of the water will decrease.The pH of a solution can be calculated using the following equation:pH = -log[H+ (aq)]where [H+ (aq)] is the concentration of H+ (aq) ions in mol/L.In this case, the concentration of H+ (aq) ions is 0.025 mol/L, so the pH of the water is:pH = -log(0.025) = 1.60Therefore, the correct answer is D) 2.39 ppm.

A Sample of Water Has Hardness Expressed as 81 Ppm