Calculating Concentration

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

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:

$concentration (g / {dm}^{3}) = \frac{mass of solute (g)}{volume of solution ({dm}^{3})}$

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

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:

$concentration (mol / {dm}^{3}) = \frac{number of moles of solute (mol)}{volume of solution ({dm}^{3})}$

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/dm³and 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.

Answer

Worked example

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol/dm³, when 80 g is dissolved in 500 cm³of water.

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

Answer

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

Examiner Tip

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

Calculating Concentration (Cambridge O Level Chemistry)

Revision Note

Units of Concentration

Calculating Concentration

Worked example

Calculating Concentration using Moles

Worked example

Worked example

Worked example

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Author: Caroline