Concentration · Year 12

In Year 12 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

  • 1 dm³ = 1 litre = 1000 cm³: divide a volume in cm³ by 1000 to get dm³.
  • concentration (g/dm³) = mass (g) ÷ volume (dm³).
  • 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

20 g of sodium chloride is dissolved in water to make 2 dm³ of solution. What is the concentration in g/dm³?

Answer: 10 g/dm³

Example 2

5 g of copper sulfate is dissolved to make 250 cm³ of solution. Find the concentration in g/dm³.

Answer: 20 g/dm³

Example 3

What mass of sugar is dissolved in 500 cm³ of a sugar solution with a concentration of 40 g/dm³?

Answer: 20 g

Example 4

Which is more concentrated: 6 g of salt in 200 cm³ of solution, or 10 g of salt in 500 cm³ of solution?

Answer: 6 g in 200 cm³ (30 g/dm³, compared with 20 g/dm³)

Example 5

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 6

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 7

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

Answer: 0.1 mol

Example 8

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 9

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 10

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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