How to calculate pH of HCl in water?

Calculate the pH of 0.1825 g of HCl dissolved in 200 mL of distilled water.?

• Answer:

  First you must convert this HCl value into mols, then into mol/L. HCl molar mass: 36.46 g/mol. 0.1825 g HCl x 1 mol HCl/ 36.46 g HCl = 5.005*10^-3 mol HCl Convert mL into L = 200 mL x 1 L/1000 mL = 0.200 L 5.055*10^-3 mol HCl/ 0.200 L = 2.50*10^-2 mol/L HCl Now you have to put it into an equation. pH is equal to -log of the concentration of hydronium (H30+) in a mixture. But in this case, we have HCl; HCl is a strong acid, and completely dissociates into its constituent elements, and doesn't form a equilibrium like a weak acid would. HCl (aq) --> H+ + Cl- Therefore, the concentration of HCl = concentration of Hydronium. pH = -log[H30] pH = -log(2.50*10^-2) pH = 1.60 The pH is 1.60 which means it's acidic, obviously since HCl is an acid.