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Concentration of Solutions

FoundationHigherCombined & TripleAQAEdexcelOCR

Get to grips with Concentration of Solutions for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Concentration links the mass or moles of solute to the volume of solution, measured in g/dm³ or mol/dm³.

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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.

Foundation

WorksheetFoundation · PDFMark SchemeFoundation · PDF

Higher

WorksheetHigher · PDFMark SchemeHigher · PDF

Topic overview

The concentration of a solution measures how much solute is dissolved in a given volume of solvent.

It can be expressed in grams per cubic decimetre, using concentration = mass ÷ volume, where volume is in dm³. It can also be expressed in moles per cubic decimetre, using concentration = moles ÷ volume.

Unit conversion is where marks are usually lost. One cubic decimetre is 1000 cm³, so a volume given in cm³ must be divided by 1000 before substituting. A 25 cm³ sample is 0.025 dm³, and forgetting that conversion makes the answer wrong by a factor of a thousand.

Revision notes

The two forms

Concentration in g/dm³ = mass in g ÷ volume in dm³.

Concentration in mol/dm³ = moles ÷ volume in dm³. The second form is used for titration calculations and is Higher tier.

Converting volumes

1 dm³ = 1000 cm³, so divide a volume in cm³ by 1000.

25 cm³ = 0.025 dm³. 250 cm³ = 0.25 dm³. Always convert before substituting into the formula.

Converting between the two

To change mol/dm³ into g/dm³, multiply by the relative formula mass.

To go the other way, divide by the Mr. This link lets you move between the two forms whichever the question gives you.

Key points

  • Concentration measures solute per unit volume.
  • Concentration in g/dm³ = mass ÷ volume.
  • Concentration in mol/dm³ = moles ÷ volume.
  • 1 dm³ = 1000 cm³.
  • Convert cm³ to dm³ by dividing by 1000.
  • Multiply mol/dm³ by Mr to get g/dm³.

Worked examples

Example 1

20 g of solute is dissolved in 500 cm³ of solution. Calculate the concentration in g/dm³. [3 marks]

Model answer

Convert the volume: 500 ÷ 1000 = 0.5 dm³ (1 mark). Concentration = mass ÷ volume = 20 ÷ 0.5 (1 mark) = 40 g/dm³ (1 mark).

Example 2

Convert 25 cm³ into dm³. [1 mark]

Model answer

25 ÷ 1000 = 0.025 dm³ (1 mark).

Example 3

A solution has a concentration of 2 mol/dm³. The Mr of the solute is 40. Calculate the concentration in g/dm³. [2 marks]

Model answer

Multiply the concentration in mol/dm³ by the relative formula mass: 2 × 40 (1 mark) = 80 g/dm³ (1 mark).

Common mistakes

  • Forgetting to convert cm³ to dm³.

    This makes the answer wrong by a factor of 1000.

  • Dividing by the Mr when converting mol/dm³ to g/dm³.

    Multiply by the Mr to go from moles to grams.

  • Using the wrong volume units in the formula.

    The formula requires dm³, so always convert first.

  • Confusing mass and moles in the formula.

    Check which form of concentration the question wants.

Exam tips

  • Convert the volume to dm³ as your first step, every time.
  • Write the formula down before substituting.
  • Check the unit asked for — g/dm³ or mol/dm³.
  • Multiply by Mr to convert moles to grams.

Key terms

Concentration
The amount of solute dissolved in a given volume.
Solute
The substance dissolved in a solvent.
Cubic decimetre
A volume of 1000 cm³, written dm³.
Solution
A solute dissolved in a solvent.

Related topics

Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.