Question 1 Medium
Calculate the concentration in g/dm³ when 3.0 g of potassium chloride is dissolved to make 600 cm³ of solution.
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Answer: 5.0 g/dm³
- 600 cm³ = 0.60 dm³.
- 3.0 ÷ 0.60 = 5.0.
Concentration and limiting reactants: GCSE Combined Science Chemistry practice questions
A second set of questions on concentration and limiting reactants. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
Calculate the mass of solute in 2.0 dm³ of a solution with a concentration of 15 g/dm³.
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Answer: 30 g
- Mass = concentration × volume.
- 15 × 2.0 = 30.
Question 3 Medium
4.0 g of sodium hydroxide is dissolved to make 100 cm³ of solution. Calculate the concentration in g/dm³.
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Answer: 40 g/dm³
- 100 cm³ = 0.10 dm³.
- Concentration = 4.0 ÷ 0.10 = 40.
Question 4 Medium
Calculate the mass of solute in 250 cm³ of a solution with a concentration of 12 g/dm³.
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Answer: 3.0 g
- 250 cm³ = 0.25 dm³.
- Mass = 12 × 0.25 = 3.0.
Question 5 Hard
A solution contains 0.50 mol of solute in 250 cm³. Calculate the concentration in mol/dm³.
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Answer: 2.0 mol/dm³
- 250 cm³ = 0.25 dm³.
- 0.50 ÷ 0.25 = 2.0.
Question 6 Hard
Calculate the number of moles of solute in 25 cm³ of a solution of concentration 0.20 mol/dm³.
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Answer: 0.0050 mol
- 25 cm³ = 0.025 dm³.
- Moles = 0.20 × 0.025 = 0.0050.
Question 7 Hard
A solution of sodium hydroxide has a concentration of 40 g/dm³. Calculate the concentration in mol/dm³. Mr of NaOH = 40.
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Answer: 1.0 mol/dm³
- Divide g/dm³ by Mr.
- 40 ÷ 40 = 1.0.
Question 8 Hard
For 2Mg + O₂ → 2MgO, 0.20 mol of magnesium reacts with 0.15 mol of oxygen. Which reactant is in excess?
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Answer: oxygen
- 0.20 mol of Mg needs 0.10 mol of O₂.
- There is 0.15 mol of O₂, so oxygen is in excess.
Question 9 Medium
A solution of potassium nitrate has a concentration of 25 g/dm³. Calculate the mass of potassium nitrate in 400 cm³ of the solution.
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Answer: 10 g
- 400 cm³ = 0.40 dm³.
- Mass = 25 × 0.40 = 10 g.
Question 10 Hard
Calculate the concentration in mol/dm³ of 20 g of sodium hydroxide dissolved to make 500 cm³ of solution. Mr of NaOH = 40.
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Answer: 1.0 mol/dm³
- Moles = 20 ÷ 40 = 0.50.
- Concentration = 0.50 ÷ 0.500 = 1.0 mol/dm³.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What does concentration measure?
Show answer
The amount of solute in a given volume of solution.
Check 2
How do you change cm³ into dm³?
Show answer
Divide by 1000.
Check 3
What is the equation for concentration in g/dm³?
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Mass in g ÷ volume in dm³.
Check 4
How do you find the mass of solute from concentration?
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Mass = concentration × volume.
Check 5
What is the equation for concentration in mol/dm³?
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Moles ÷ volume in dm³.
Check 6
How do you convert g/dm³ into mol/dm³?
Show answer
Divide by the relative formula mass.
Check 7
How do you convert mol/dm³ into g/dm³?
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Multiply by the relative formula mass.
Check 8
What is 500 cm³ in dm³?
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0.5 dm³.
Check 9
What is a limiting reactant?
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The reactant that is used up first.
Check 10
What is a reactant in excess?
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A reactant with some left over when the reaction stops.
Check 11
What decides the amount of product?
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The limiting reactant.
Check 12
How do you find the limiting reactant?
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Convert each mass to moles and compare using the equation ratio.
Check 13
Why is a reactant used in excess?
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To make sure the other reactant is used up completely.
Check 14
What is a more concentrated solution?
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One with more solute in the same volume.
Check 15
How many moles are in 25 cm³ of 1.0 mol/dm³ solution?
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0.025 mol.
Check 16
What is the unit for concentration?
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g/dm³ or mol/dm³.
Check 17
What is a solute?
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The substance that dissolves in a liquid.
Check 18
What is a mole?
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The unit chemists use to measure the amount of a substance.
Check 19
What is 2000 cm³ in dm³?
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2 dm³.
Check 20
What does Mr stand for?
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Relative formula mass, the mass of one mole in grams.
Check 21
How do you find moles from concentration and volume?
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Moles = concentration × volume in dm³.
Check 22
Why must volume be in dm³ in a concentration calculation?
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Because concentration units are per dm³, as in g/dm³ and mol/dm³.
Check 23
Which reactant is usually the limiting one in industry?
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The more expensive one, so none of it is wasted.
Check 24
What is the mass of 1 mole of sodium hydroxide, NaOH?
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40 g, because its Mr is 40.
Check 25
How do you find which reactant is limiting?
Show answer
Work out the moles of each, use the equation ratio, and find which one runs out first.
Keep going with Concentration and limiting reactants
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