GCSE Chemistry revision

Concentration of solutions

Use of amount of substance in relation to masses of pure substances

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What is Concentration?

Concentration measures how much solute is dissolved in a given volume of solution,
expressed in grams per decimetre cubed (g dm³).
One decimetre cubed (dm³) is equal to 1,000 cm³, which is the same volume as one litre of water.
The unit g dm^-3 can also be written as g per dm³, where the negative index ^-3 means the same as 'per'.

1. What is Concentration? AQA

Notes

Concentration measures how much solute is dissolved in a given volume of solution,
expressed in grams per decimetre cubed (g dm³).
One decimetre cubed (dm³) is equal to 1,000 cm³, which is the same volume as one litre of water.
The unit g dm^-3 can also be written as g per dm³, where the negative index ^-3 means the same as 'per'.

The Concentration Formula

The equation for concentration is: Concentration = (Mass / Volume) where mass is in grams (g) and volume is in dm³.
A formula triangle can be used to rearrange the equation: cover the quantity you want to find and read the remaining two — divide if they are above/below each other, multiply if side by side.
The three rearrangements are: C = (m / V), m = C × V, and V = (m / C).

2. The Concentration Formula AQA

Notes

The equation for concentration is: Concentration = (Mass / Volume) where mass is in grams (g) and volume is in dm³.
A formula triangle can be used to rearrange the equation: cover the quantity you want to find and read the remaining two — divide if they are above/below each other, multiply if side by side.
The three rearrangements are: C = (m / V), m = C × V, and V = (m / C).

Converting Units Before Calculating

Volume must always be in dm³ before using the concentration formula, so cm³ must be converted by dividing by 1,000.
For example, 480 cm³ divided by 1,000 gives 0.48 dm³, which can then be used directly in the formula.
Always check the units given in a question, as being given cm³ instead of dm³ (or kg instead of g) is a common exam trap.

3. Converting Units Before Calculating AQA

Notes

Volume must always be in dm³ before using the concentration formula, so cm³ must be converted by dividing by 1,000.
For example, 480 cm³ divided by 1,000 gives 0.48 dm³, which can then be used directly in the formula.
Always check the units given in a question, as being given cm³ instead of dm³ (or kg instead of g) is a common exam trap.

Worked Example: Finding Concentration

To find the concentration of 40 g of sodium hydroxide dissolved in 2.5 dm³ of water: C = (40 / 2.5) = 16 g dm⁻³.
To find the concentration of 24 g of copper sulfate in 480 cm³ of water, first convert to 0.48 dm³, then: C = (24 / 0.48) = 50 g dm⁻³.

Worked Example: Finding Mass

When concentration and volume are known, mass can be calculated using: m = C ×V.
For a 400 cm³ sample of hydrochloric acid with a concentration of 20 g dm³, convert 400cm³ to 0.4 dm³ first.
Then calculate: m = 20 × 0.4 = 8 g, meaning 8 g of hydrogen chloride is present in the solution.

5. Worked Example: Finding Mass AQA

Notes

When concentration and volume are known, mass can be calculated using: m = C ×V.
For a 400 cm³ sample of hydrochloric acid with a concentration of 20 g dm³, convert 400cm³ to 0.4 dm³ first.
Then calculate: m = 20 × 0.4 = 8 g, meaning 8 g of hydrogen chloride is present in the solution.