Units of concentration

• A solute is a solid substance that dissolves into a liquid 

  • The amount of solute can be expressed in grams (g) or moles (mol)

• A solvent is the liquid that a solute dissolves in 

  • The amount / volume of a solvent is measured in cm3 or dm3

• Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent

• A solution is the mixture formed when a solute dissolves in a solvent 

  • The amount / volume of a solution measured in cm3 or dm3 

• Concentration 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

  • Concentration is sometimes commonly referred to as strength

    • For example, dissolving more coffee in hot water results in a stronger coffee

• Typically, concentration is expressed in terms of the amount of substance per dm3  

  • Therefore, the units of concentration are:

    • g / dm3 

    • mol / dm3

Calculating concentration using mass 

The general formula to calculate the concentration in g / dm3 is:

Concentration can also 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

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:

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