Calculating Concentration (Cambridge (CIE) O Level Chemistry): Revision Note

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 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

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:

• 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:

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