Reversing the calculations

3 marks · Normal

Citric acid reacts with sodium hydroxide as follows:

Calculate the volume, in cm³, of 0.300 mol/dm³ sodium hydroxide solution required to completely neutralise 20.0 cm³ of 0.200 mol/dm³ citric acid solution.

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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90.0 cm³ (±0.5)

Also accept: 89.9 cm³; 90 cm³

Common mistakes: Failing to multiply by stoichiometric ratio (3:1); Not converting units between cm³ and dm³

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Calculate moles of citric acid from its volume and concentration.

M2 · 1 mark · Use the chemical equation to determine the moles of NaOH required using the 3:1 molar ratio. (follow-through)

A1 · 1 mark · Correctly calculate the volume of sodium hydroxide solution and express in cm³ to an appropriate number of significant figures. (follow-through)

Worked solution

Step 1: Calculate the number of moles of citric acid present.

Step 2: Use the stoichiometry of the equation. The equation shows 1 mol citric acid reacts with 3 mol NaOH, so:

Step 3: Rearranging the concentration equation to find the volume of NaOH (in dm³):

Convert this to cm³:

Final answer: 40.0 cm³ of 0.300 mol/dm³ sodium hydroxide solution is required.