Concentrations of solutions

• In chemistry the mole is the main unit of the amount of substance that is used.
• The number of moles of solute present in a volume needs to be known by chemists (the concentration of the solution).
• The concentration of a solution is measured in mols per cubic decimetre (mol dm^-3).
• One mole of any solute in a 1 litre volume has a concentration of 1 mol dm^-3, for example 6.02 x 10²³ particles of NaOH in a one litre solution, will have a concentration of 1 mol dm^-3.

• The concentration of a volume is worked out by dividing the number of moles by the volume:

Example:

We have 250 cm³ of a solution of sodium hydroxide whose concentration is 2 mole dm^-3. How many moles of sodium hydroxide do we have?

Rearrange the formula: Number of Moles (N)= Concentration (C) x Volume (V in dm³)
Change the units: 250 cm³ is the same as 0.25 dm³ (by dividing by 1000).
Substitute the values into the formula:
Number of Moles = 2 x 0.25
=0.5 moles

Useful books for revision:

Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)

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