Question 1 Medium
In 2Mg + O₂ → 2MgO, 12 g of magnesium burns. Calculate the mass of magnesium oxide formed. Ar of Mg = 24 and Mr of MgO = 40.
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Answer: 20 g
- Moles of Mg = 12 ÷ 24 = 0.50.
- Moles of MgO = 0.50, so mass = 0.50 × 40 = 20 g.
The mole and reacting masses: GCSE Combined Science Chemistry practice questions
A second set of questions on the mole and reacting masses. 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
CaCO₃ → CaO + CO₂. Calculate the mass of carbon dioxide made when 20 g of calcium carbonate decomposes. Mr of CaCO₃ = 100 and Mr of CO₂ = 44.
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Answer: 8.8 g
- Moles of CaCO₃ = 20 ÷ 100 = 0.20.
- The ratio is 1 : 1, so mass of CO₂ = 0.20 × 44 = 8.8 g.
Question 3 Medium
Calculate the mass of 0.20 mol of calcium hydroxide, Ca(OH)₂. Mr = 74.
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Answer: 14.8 g
- Mass = moles × Mr.
- 0.20 × 74 = 14.8 g.
Question 4 Medium
In 2H₂ + O₂ → 2H₂O, how many moles of water form from 0.40 mol of oxygen?
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Answer: 0.80 mol
- The ratio of O₂ to H₂O is 1 : 2.
- Double 0.40 mol is 0.80 mol.
Question 5 Hard
Calculate the number of atoms in 0.50 mol of carbon. Give the answer in standard form.
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Answer: 3.01 × 10²³
- Atoms = moles × 6.02 × 10²³.
- 0.50 × 6.02 × 10²³ = 3.01 × 10²³.
Question 6 Hard
Iron is made in the reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂. Calculate the mass of iron made from 80 g of iron(III) oxide. Mr of Fe₂O₃ = 160 and Ar of Fe = 56.
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Answer: 56 g
- Moles of Fe₂O₃ = 80 ÷ 160 = 0.50.
- Moles of Fe = 2 × 0.50 = 1.0.
- Mass = 1.0 × 56 = 56 g.
Question 7 Hard
2.4 g of magnesium reacts with 1.6 g of oxygen. Work out the simplest formula of the product. Ar of Mg = 24 and O = 16.
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Answer: MgO
- Moles of Mg = 2.4 ÷ 24 = 0.10.
- Moles of O = 1.6 ÷ 16 = 0.10, so the ratio is 1 : 1.
Question 8 Hard
Calculate the mass of aluminium oxide formed when 5.4 g of aluminium burns in oxygen: 4Al + 3O₂ → 2Al₂O₃. Ar of Al = 27, Mr of Al₂O₃ = 102.
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Answer: 10.2 g
- Moles of Al = 5.4 ÷ 27 = 0.20.
- Moles of Al₂O₃ = 0.10.
- Mass = 0.10 × 102 = 10.2 g.
Question 9 Medium
Calculate the number of moles in 5.6 g of iron. Ar of Fe = 56.
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Answer: 0.10 mol
- Moles = mass ÷ Ar.
- 5.6 ÷ 56 = 0.10.
Question 10 Hard
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Calculate the mass of carbon dioxide made when 21 g of sodium hydrogencarbonate decomposes. Mr of NaHCO₃ = 84 and CO₂ = 44.
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Answer: 5.5 g
- Moles of NaHCO₃ = 21 ÷ 84 = 0.25.
- The ratio NaHCO₃ : CO₂ is 2 : 1, so moles of CO₂ = 0.125.
- Mass = 0.125 × 44 = 5.5 g.
Question 11 Hard
0.30 mol of hydrogen reacts with 0.10 mol of nitrogen to make 0.20 mol of ammonia. Write the simplest ratio of moles H₂ : N₂ : NH₃.
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Answer: 3 : 1 : 2
- Divide each amount by the smallest, 0.10.
- 0.30 : 0.10 : 0.20 = 3 : 1 : 2.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
How many particles are in one mole?
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6.02 × 10²³.
Check 2
What is the Avogadro constant?
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6.02 × 10²³ per mole.
Check 3
What is the equation linking moles, mass and Mr?
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Moles = mass ÷ Mr.
Check 4
How do you find mass from moles?
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Multiply moles by Mr.
Check 5
What is the mass of one mole of carbon atoms?
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12 g.
Check 6
What is the mass of one mole of water?
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18 g.
Check 7
What do the big numbers in an equation show?
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The ratio of moles of each substance.
Check 8
What is the first step in a reacting mass calculation?
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Write the balanced equation.
Check 9
What is the second step in a reacting mass calculation?
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Convert the known mass into moles.
Check 10
How do you convert moles of the known substance into moles of the wanted one?
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Use the ratio of the big numbers in the equation.
Check 11
How many atoms are in 1 mole of oxygen molecules, O₂?
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2 × 6.02 × 10²³.
Check 12
How do you find the simplest ratio from moles?
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Divide each by the smallest number of moles.
Check 13
What is the mass of 0.5 mol of NaOH?
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20 g, because Mr = 40.
Check 14
How many moles are in 12 g of magnesium?
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0.5 mol, because Ar = 24.
Check 15
What are the units of molar mass?
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g per mol.
Check 16
What should you give with every calculation answer?
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The correct unit.
Check 17
What is a mole in chemistry?
Show answer
The amount of a substance that contains 6.02 × 10²³ particles.
Check 18
How many atoms are in 1 mole of carbon atoms?
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6.02 × 10²³.
Check 19
What equation gives the number of particles from moles?
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Number of particles = moles × 6.02 × 10²³.
Check 20
How many moles are in 36 g of water?
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2 mol, because Mr = 18.
Check 21
What is the mass of 2 mol of carbon dioxide?
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88 g, because Mr = 44.
Check 22
What is the third step in a reacting mass calculation?
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Use the ratio from the equation to find the moles of the other substance.
Check 23
What is the last step in a reacting mass calculation?
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Turn the moles of the wanted substance into mass using Mr.
Check 24
How many atoms are in one molecule of H₂O?
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Three.
Check 25
How do you find the balancing numbers of an equation from reacting masses?
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Convert each mass to moles, divide by the smallest, and use the simplest whole-number ratio.
Check 26
What is the mass of one mole of water in grams?
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18 g, because the Mr of H₂O is 18.
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