100 cm3 of water is added to 20 cm3 of 0.100 mol dm–3 sulfuric acid. Calculate the pH of the mixture

Answer 1

By adding 100 cm³ of water, you are diluting the existing sulfuric acid solution, which will end up lowering the concentration from 0.100 mol dm^-3.

Use the dilution equation to first calculate the molarity of the diluted solution.

M₁V₁ = M₂V₂

V₂ = 100 + 20 = 120 cm³

M₂ = M₁V₁ / V₂ = (0.100 x 20) / 120 = 0.0167 mol dm^-3

Since H₂SO₄ is a strong acid, it ionizes completely in solution. The dissociation reaction is written as:

H₂SO₄ → 2H⁺ + SO₄^2-

Since sulfuric acid is a diprotic acid, it ionizes to form 2 moles of hydrogen ions. The concentration of H⁺ is therefore, 2 x 0.0167 mol dm^-3 = 0.0334 mol dm^-3

Since we know [H⁺], the pH can be calculated using the equation,

pH = - log [H⁺] = - log 0.0334 = 1.48