Concentrations and gas volumes Cambridge IGCSE Chemistry revision
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In plain words
A concentration tells you how much is dissolved in each litre (dm³) of a solution. For gases there's a neat shortcut: at room temperature and pressure, a mole of any gas at all takes up the same space, 24 dm³.
Three things to know
- Concentration (mol/dm³) = moles ÷ volume in dm³. To change cm³ into dm³, divide by 1000.
- One mole of any gas takes up 24 dm³ (24 000 cm³) at room temperature and pressure.
- In a titration: moles = concentration × volume. Use the equation to find the moles of the other solution, then its concentration.
Worked example
What volume of hydrogen, at room temperature and pressure, is made from 0.1 mol of magnesium? Mg + 2HCl → MgCl₂ + H₂.
- The equation shows 1 mol of magnesium gives 1 mol of hydrogen, so 0.1 mol is made.
- Volume = moles × 24 dm³.
- = 2.4 dm³.
Tips and tricks
- Change cm³ to dm³ before using a concentration: divide by 1000.
- In a titration, find the moles of the solution you know everything about first.
It lands in your notebook with its questions as flashcards.
Concentrations and gas volumes: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
2 mol of solute is dissolved to make 4 dm³ of solution. What is the concentration?
2 ÷ 4.
What volume does 2 mol of any gas occupy at room temperature and pressure?
2 × 24.
What is 250 cm³ in dm³?
Divide by 1000.
How many moles of gas are there in 12 dm³ at room temperature and pressure?
12 ÷ 24.
How many moles of solute are in 100 cm³ of a 0.5 mol/dm³ solution?
0.5 × 0.100.
40 g of sodium hydroxide (Mr = 40) is dissolved to make 2 dm³ of solution. What is its concentration?
40 g is 1 mol; 1 ÷ 2.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Concentrations and gas volumes
Cambridge IGCSE Chemistry 0620 · 9 marks · papermunch.org
Name ______________________________ Date ______________
0.5 mol of solute is dissolved to make 250 cm³ of solution. Calculate its concentration.[2]
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2.0 mol/dm³. 0.5 ÷ 0.250.
Calculate the volume of 0.25 mol of carbon dioxide at room temperature and pressure.[2]
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6.0 dm³. 0.25 × 24.
25.0 cm³ of sodium hydroxide solution is exactly neutralised by 20.0 cm³ of 0.100 mol/dm³ hydrochloric acid. NaOH + HCl → NaCl + H₂O. Calculate the concentration of the sodium hydroxide.[3]
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0.0800 mol/dm³. Moles of HCl = 0.100 × 0.0200 = 0.00200, so moles of NaOH = 0.00200; 0.00200 ÷ 0.0250.
Calculate the number of moles of solute in 50 cm³ of a 2.0 mol/dm³ solution.[2]
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0.10 mol. 2.0 × 0.050.
Answers: Concentrations and gas volumes
- 1. 2.0 mol/dm³. 0.5 ÷ 0.250.
- 2. 6.0 dm³. 0.25 × 24.
- 3. 0.0800 mol/dm³. Moles of HCl = 0.100 × 0.0200 = 0.00200, so moles of NaOH = 0.00200; 0.00200 ÷ 0.0250.
- 4. 0.10 mol. 2.0 × 0.050.