Concentration · Year 13

In Year 13 you get fluent with: concentration in mol/dm³; diluting a solution; changing between g/dm³ and mol/dm³.

The concentration of a solution tells you how much solute is dissolved in each dm³ (litre) of it. In g/dm³ it is the mass in each dm³; in mol/dm³ it is the number of moles. Multiply by the Mᵣ to go from mol/dm³ to g/dm³. Adding water dilutes a solution: the moles stay the same in a bigger volume.

Key facts

  • concentration (mol/dm³) = moles ÷ volume (dm³), so moles = concentration × volume.
  • To change mol/dm³ into g/dm³, multiply by the Mᵣ.
  • Diluting with water keeps the moles of solute the same, so the concentration falls as the volume grows.

Worked examples

Example 1

4 g of sodium hydroxide is dissolved to make 250 cm³ of solution. Find its concentration in g/dm³ and in mol/dm³. (Mᵣ of NaOH = 40)

Answer: 16 g/dm³; 0.4 mol/dm³

Example 2

0.5 mol of hydrogen chloride is dissolved to make 2 dm³ of hydrochloric acid. What is its concentration in mol/dm³, and how many moles of HCl are in 100 cm³ of it?

Answer: 0.25 mol/dm³; 0.025 mol

Example 3

How many moles of solute are in 50 cm³ of a 2 mol/dm³ solution?

Answer: 0.1 mol

Example 4

10 cm³ of 1 mol/dm³ acid is diluted with water to 200 cm³. What is the new concentration in mol/dm³?

Answer: 0.05 mol/dm³

Example 5

A solution of chem NaOH has a concentration of 0.1 mol/dm³. What is its concentration in g/dm³? (Aᵣ: Na = 23, O = 16, H = 1)

Answer: 4 g/dm³

Example 6

A solution of chem NaOH contains 80 g/dm³. What is its concentration in mol/dm³? (Aᵣ: Na = 23, O = 16, H = 1)

Answer: 2 mol/dm³

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