How many particles are there in one mole?

6.02 × 10²³. This is the Avogadro constant.

CaCO₃ → CaO + CO₂. What mass of calcium oxide is made from 100 g of calcium carbonate? (Mr: CaCO₃ = 100, CaO = 56)

56 g. 1 mol of CaCO₃ gives 1 mol of CaO.

How many moles of hydrogen atoms are there in 1 mol of methane, CH₄?

4. Each molecule contains four hydrogen atoms.

The mole and reacting masses · Cambridge IGCSE Co-ordinated Sciences (9

In plain words

Atoms are too small to count, so chemists count them in huge batches, the way eggs are counted in dozens. One batch is a mole. Usefully, a mole of any substance weighs its relative formula mass in grams: a mole of water (Mr 18) is 18 g.

Three things to know

One mole of anything contains 6.02 × 10²³ particles (the Avogadro constant). The mass of one mole is the Ar or Mr in grams.
Moles = mass ÷ molar mass. Rearranged: mass = moles × molar mass.
The numbers in a balanced equation give the ratio of moles reacting. Percentage yield = actual mass ÷ theoretical mass × 100.

Worked example

What mass of carbon dioxide is made when 6 g of carbon burns? C + O₂ → CO₂. (C = 12, O = 16)

Moles of carbon = mass ÷ Ar = 6 ÷ 12 = 0.5 mol.
The equation shows 1 mol of carbon gives 1 mol of carbon dioxide, so 0.5 mol is made.
Mass = moles × Mr = 0.5 × 44 = 22 g.

Tips and tricks

Every reacting-mass question is three steps: mass to moles, use the ratio in the equation, moles back to mass.
Use the big numbers in the equation for the ratio, never the masses.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

The mole and reacting masses: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

How many moles are there in 36 g of water? (Mr = 18)
- 0.5
- 1
- 2 (the answer)
- 18
36 ÷ 18.
What is the mass of 2 mol of carbon? (Ar = 12)
- 6 g
- 12 g
- 14 g
- 24 g (the answer)
2 × 12.
How many particles are there in one mole?
- 6.02 × 10²
- 6.02 × 10¹³
- 6.02 × 10²³ (the answer)
- 6.02 × 10³²
This is the Avogadro constant.
CaCO₃ → CaO + CO₂. What mass of calcium oxide is made from 100 g of calcium carbonate? (Mr: CaCO₃ = 100, CaO = 56)
- 44 g
- 56 g (the answer)
- 100 g
- 156 g
1 mol of CaCO₃ gives 1 mol of CaO.
A reaction gives 4 g of product. The theoretical yield is 5 g. What is the percentage yield?
- 20%
- 45%
- 80% (the answer)
- 125%
4 ÷ 5 × 100.
How many moles of hydrogen atoms are there in 1 mol of methane, CH₄?
- 1
- 2
- 4 (the answer)
- 5
Each molecule contains four hydrogen atoms.

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.

The mole and reacting masses

Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 9 marks · papermunch.org

Name ______________________________ Date ______________

Calculate the number of moles in 22 g of carbon dioxide. (Mr = 44)[2]
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0.5 mol. 22 ÷ 44.
I got this one
Calculate the mass of 0.25 mol of sodium hydroxide. (Mr = 40)[2]
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10 g. 0.25 × 40.
I got this one
2Mg + O₂ → 2MgO. Calculate the mass of magnesium oxide made from 12 g of magnesium. (Mg = 24, O = 16)[3]
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20 g. 12 ÷ 24 = 0.5 mol of Mg, which gives 0.5 mol of MgO; 0.5 × 40.
I got this one
A reaction should give 8.0 g of product but only 6.0 g is collected. Calculate the percentage yield.[2]
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75%. 6.0 ÷ 8.0 × 100.
I got this one

Answers: The mole and reacting masses

1. 0.5 mol. 22 ÷ 44.
2. 10 g. 0.25 × 40.
3. 20 g. 12 ÷ 24 = 0.5 mol of Mg, which gives 0.5 mol of MgO; 0.5 × 40.
4. 75%. 6.0 ÷ 8.0 × 100.