Titration Calculations - 1

Not every titration reaction has a 1 : 1 ratio. Sulfuric acid is diprotic: each H₂SO₄ releases two H⁺ ions, so it needs two moles of sodium hydroxide.H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)The ratio H₂SO₄ : NaOH is 1 : 2. Getting this step the wrong way round is the most common error, so write the ratio out in words before you use it.Worked example: 25.0 cm³ of 0.150 mol dm^-3 sodium hydroxide was pipetted into a conical flask and titrated with sulfuric acid. The mean titre was 18.75 cm³. Calculate the concentration of the sulfuric acid.Step 1: amount of NaOH = 0.150 × (25.0 ÷ 1000) = 3.75 × 10^-3 molStep 2: 2 mol NaOH reacts with 1 mol H₂SO₄, so amount of H₂SO₄ = 3.75 × 10^-3 ÷ 2 = 1.875 × 10^-3 molStep 3: concentration of H₂SO₄ = 1.875 × 10^-3 ÷ (18.75 ÷ 1000) = 0.100 mol dm^-3A useful check: the acid titre is smaller than the volume of alkali even though the acid is more dilute. That makes sense, because each mole of sulfuric acid neutralises twice as much alkali as a mole of hydrochloric acid would.The same method works for any ratio: always go through moles and the balanced equation.