Concentrations in mol dm-3

Higher Tier

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 dm^–3
- mol dm^–3

What is 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 dm^–³ 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) is the same as 1 litre
- 1 decimetre cubed (dm³) = 1000 cm³
You may be given data in a question which needs to be converted from cm³ to dm³ or the other way around

Conversion of cm³ and dm³

unit-conversion

To go from cm³ to dm³divide by 1000. To go from dm³ to cm³multiply by 1000

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:

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

We can modify the concentration formula to include moles
- The units in the answer can be written as mol dm^–3

Some students find formula triangles help them to understand the relationship:

Diagram to show the relationship between moles, concentration and volume

Concentration moles formula triangle, downloadable IB Chemistry revision notes

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

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

Step 1: Write down the given information

Mass = 10 g
Volume of solution = 2 dm³

Step 2: Calculate concentration in g dm^–3

Concentration = $\frac{mass (g)}{volume ({dm}^{3})}$ = $\frac{10 g}{2 {dm}^{3}}$ = 5 g dm^–3

Worked example

Calculate the amount of solute, in moles, present in 2.5 dm³ of a solution whose concentration is 0.2 mol dm^–³.

Answer

Step 1: Write down the information you are given in the question:

Concentration of solution: 0.2 mol dm^–³
Volume of solution: 2.5 dm³

Step 2: Calculate the number of moles

Moles = concentration x volume
Moles = 0.2 x 2.5 = 0.5 mol

Exam Tip

You may not be given the equation in an exam, so make sure you are able to recall it.

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

Concentrations in mol dm-3 (WJEC GCSE Chemistry)

Revision Note