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The concentration of a solution can be shown in g/dm³ or mol/dm³. It is often more useful to know the concentration of a reactant in mol/dm³ so that the amount of reactant in a given volume can be calculated.

Concentration using moles

The concentration of a solution can be calculated using:

• the amount of dissolvedsolute in moles, mol
• the volume of solution (or solvent) in cubic decimetres, dm³

Example

0.5 mol of sodium hydroxide is dissolved in 2 dm³ of water. Calculate the concentration of the sodium hydroxide solution formed.

Concentration = 0.25 mol/dm³

Volume units

Volumes used in concentration calculations must be in dm³, not in cm³. It is useful to know that 1 dm³ = 1000 cm³. This means:

• divide by 1000 to convert from cm³ to dm³
• multiply by 1000 to convert from dm³ to cm³

For example, 250 cm³ is 0.25 dm³ (250 ÷ 1000). It is often easiest to convert from cm³ to dm³ before continuing with a concentration calculation.

Converting between units

The relative formula mass of the solute is used to convert between mol/dm³ and g/dm³:

• to convert from mol/dm³ to g/dm³, multiply by the relative formula mass
• to convert from g/dm³ to mol/dm³, divide by the relative formula mass

Remember: the molar mass is the A_r or M_r in grams per mol.

Example

Calculate the concentration of 0.1 mol/dm³ sodium hydroxide solution in g/dm³. (M_r of NaOH = 40)

Concentration = 0.1 × 40

= 4 g/dm³