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O LevelChemistryCambridgeRevision Notes3. Stoichiometry3.3 The Mole & the Avogadro ConstantCalculating Concentration

Calculating Concentration (Cambridge O Level Chemistry)

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Units of Concentration

  • A solid substance that dissolves in a liquid is called a solute, the liquid is called a solvent and the two when mixed together form a solution
  • Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
  • Concentration simply refers to the amount of solute there is in a specific volume of the solvent
  • The greater the amount of solute in a given volume, the greater the concentration
  • The amount of solute can be expressed in grams or moles
  • Typically, concentration is expressed in terms of the amount of substance per dm3, therefore the units of concentration are either:     g/dm3  or mol/dm3

Calculating Concentration

  • Concentration refers to the amount of solute there is in a specific volume of the solvent
  • A general formula to calculate the concentration in g/dm3 is:

concentration space left parenthesis straight g divided by dm cubed right parenthesis space equals space fraction numerator mass space of space solute space left parenthesis straight g right parenthesis over denominator volume space of space solution space left parenthesis dm cubed right parenthesis end fraction

  • Concentration can be measured in grams per cubic decimetre
  •  1 decimetre cubed (dm3) = 1000 cm3
    • 1 decimetre cubed (dm3) is the same as 1 litre

  • You may be given data in a question which needs to be converted from cm3 to dm3 or the other way around
    • To go from cm3 to dm3 :
      • Divide by 1000
    • To go from dm3 to cm3 :
      • Multiply by 1000

Worked example

A student dissolved 10 g of sodium hydroxide, NaOH, in 2 dm3 of distilled water. Calculate the concentration of the solution in g/dm3.

Answer

Concentration Calculations WE, downloadable IGCSE & GCSE Chemistry revision notes

Calculating Concentration using Moles

  • 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 therefore be expressed in moles per decimetre cubed and calculated using the following equation:

concentration space left parenthesis mol divided by dm cubed right parenthesis space equals space fraction numerator number space of space moles space of space solute space left parenthesis mol right parenthesis over denominator volume space of space solution space left parenthesis dm cubed right parenthesis end fraction

  • We can modify the concentration formula to include moles
    • The units in the answer can be written as mol/dm3 (this can also be written as mol dm-3)
  • You may have to convert from g/dm3 into mol/dm3 and vice versa depending on the question
    • To go from g/dm3 to mol/dm3 
      • Divide by the molar mass in grams
    • To go from mol/dm3 to g/dm3
      • Multiply by the molar mass in grams
  • Some students find formula triangles help them to understand the relationship:

Concentration moles formula triangle, downloadable IB Chemistry revision notes

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

Worked example

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

AnswerCalculating Concentrations WE1, downloadable IGCSE & GCSE Chemistry revision notes

Worked example

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol/dm3, when 80 g is dissolved in 500 cm3 of water.

Relative atomic masses, Ar:  Na= 23;   H= 1;   O= 16

Answer

3-2-4-conc-naoh

Worked example

25.0 cm3 of 0.050 mol/dm3 sodium carbonate was completely neutralised by 20.00 cm3 of dilute hydrochloric acid. Calculate the concentration in mol/dm3 of the hydrochloric acid.

Answer

3-2-4-conc-hcl

Examiner Tip

Remember to always convert the units from cm3 to dm3 by dividing by 1000.

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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.