The standard calculation

3 marks · Normal

A student carries out a titration to find the concentration of a sulfuric acid solution. She adds exactly 25.0 cm³ of sodium hydroxide solution of known concentration, 0.150 mol/dm³, to a conical flask. She slowly adds sulfuric acid from a burette, using methyl orange as an indicator, until the indicator just changes colour. She records that she has used 30.0 cm³ of sulfuric acid to reach the endpoint. The balanced chemical equation for the reaction is:

Explain carefully how you would use these results to calculate the concentration of the sulfuric acid.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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Must show: Calculate moles of NaOH from volume and concentration, Relate moles of NaOH to H2SO4 using equation ratio (2:1), Calculate concentration of H2SO4 using moles and volume

Also accept: Stepwise calculation explanations that include all logical steps and correct application of the chemical equation

Common mistakes: Not dividing moles of NaOH by 2 to get moles of H2SO4; Forgetting to convert cm^3 to dm^3; Incorrect use of equation ratio

Show mark scheme and worked solution

Marking points

M1 · 1 mark · States or shows how to calculate moles of NaOH using the given concentration and volume (e.g., moles = volume in dm³ × concentration).

M2 · 1 mark · Uses the chemical equation to relate the moles of NaOH to moles of H2SO4 (correctly applies the 2:1 mole ratio).

M3 · 1 mark · Describes or shows how to calculate the concentration of H2SO4 using moles and the volume used (moles / volume in dm³).

Worked solution

Step 1: Calculate the moles of NaOH used: volume is 25.0 cm³ = 0.0250 dm³; moles = 0.0250 dm³ × 0.150 mol/dm³ = 0.00375 mol.

Step 2: Use the chemical equation: 1 mol H2SO4 reacts with 2 mol NaOH, so: moles of H2SO4 = 0.00375 mol NaOH ÷ 2 = 0.001875 mol H2SO4.

Step 3: Calculate the concentration of H2SO4: volume used = 30.0 cm³ = 0.0300 dm³; concentration = 0.001875 mol / 0.0300 dm³ = 0.0625 mol/dm³.