AQA GCSE Chemistry

Revision Notes

3.4.1 Using Concentrations in mol/dm3

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Concentration, Moles & Volume

Concentration, moles and volume

Higher Tier Only

  • It is more useful to a chemist to express concentration in terms of moles per unit volume rather than mass per unit volume
  • Concentration can therfore be expressed in moles per decimetre cubed
  • We can modify the concentration formula to include moles
    • The units in the answer can be written as mol dm-3 or mol / dm3:

  • You may have to convert from g dm-3 into mol dm-3 and vice versa depending on the question
    • To go from g dm-3 to mol dm-3:
      • Divide by the molar mass in grams

    • To go from mol dm-3 to g dm-3:
      • Multiply by the molar mass in grams

Exam Tip

Don't forget your unit conversions:

To go from cm3 to dm3 : divide by 1000

To go from dm3 to cm3 : multiply by 1000

Calculating Concentration from Reacting Solutions

  • Solving problems on concentrations involves carefully working out moles and volumes in the correct units and applying the concentration formula
  • Some students find formula triangles help them to understand the relaitonship:

Concentration moles formula triangle, downloadable IB Chemistry revision notes

The concentration-moles formula triangle can help you solve these problems

  • The following examples show how to do this step-by-step

Worked example

Example 1

Calculate the amount of solute, in moles, present in 2.5 dm3 of a solution whose concentration is 0.2 mol dm-3.

Answer:

Calculating Concentrations WE1, downloadable IGCSE & GCSE Chemistry revision notes

Worked example

Example 2

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol dm-3, when 80 g is dissolved in 500 cm3 of water.(Na= 23, H= 1, O= 16)

Answer:

Calculating Concentrations WE2, downloadable IGCSE & GCSE Chemistry revision notes

Titration Calculations

  • If the concentration of one of the reactants is known (either the acid or the base), then the exact volumes from a titration along with the balanced chemical equation for the reaction can be used to calculate the concentration of the other reactant

Worked example

Example 3

25.0 cm3 of a solution of 0.05 mol dm-3  sodium carbonate was completely neutralised by 20.00 cm3 of dilute hydrochloric acid. Calculate the concentration of the hydrochloric acid in mol dm-3.

Answer:

Exam Tip

You are not given the concentration-moles formula triangle in exams so you have to learn it. It is a good idea to write it down before you start a problem, so you get all the parts in the correct place.

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