Question 1 Medium
9.7 g of zinc oxide is made when 7.8 g of zinc burns in oxygen. What mass of oxygen reacted? Give your answer in grams to 2 significant figures.
Show answer
Answer: 1.9 g
- Mass of oxygen = 9.7 − 7.8.
- 9.7 − 7.8 = 1.9 g.
Relative formula mass and conservation of mass: GCSE Combined Science Chemistry practice questions
A second set of questions on relative formula mass and conservation of mass. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
More questions
Question 2 Medium
A student heats 8.4 g of sodium hydrogencarbonate in an open dish. 5.3 g of solid is left. Calculate the mass of gas and water lost.
Show answer
Answer: 3.1 g
- The mass lost is the difference.
- 8.4 − 5.3 = 3.1 g.
Question 3 Medium
Calculate the relative formula mass of magnesium hydroxide, Mg(OH)₂. Ar of Mg = 24, O = 16 and H = 1.
Show answer
Answer: 58
- O and H in the bracket: 16 + 1 = 17.
- 24 + 2 × 17 = 58.
Question 4 Medium
3.6 g of magnesium burns in air. The white solid formed has a mass of 6.0 g. What mass of oxygen joined the magnesium? Give your answer in grams.
Show answer
Answer: 2.4 g
- The extra mass is the oxygen.
- 6.0 − 3.6 = 2.4 g.
Question 5 Hard
Calculate the relative formula mass of aluminium sulfate, Al₂(SO₄)₃. Ar of Al = 27, S = 32 and O = 16.
Show answer
Answer: 342
- Al: 2 × 27 = 54.
- SO₄ is 32 + 64 = 96, and there are 3, giving 288.
- 54 + 288 = 342.
Question 6 Hard
Show that the equation 2H₂ + O₂ → 2H₂O is balanced using relative formula masses. Give the total Mr of the products. Ar of H = 1 and O = 16.
Show answer
Answer: 36
- Reactants: 2 × 2 + 32 = 36.
- Products: 2 × 18 = 36.
Question 7 Hard
Calculate the relative formula mass of copper(II) sulfate crystals, CuSO₄·5H₂O. Ar of Cu = 63.5, S = 32, O = 16 and H = 1.
Show answer
Answer: 249.5
- CuSO₄ = 63.5 + 32 + 64 = 159.5.
- 5H₂O = 5 × 18 = 90.
- 159.5 + 90 = 249.5.
Question 8 Hard
Explain why a student finds the mass of an open beaker falls when marble chips react with acid.
Show answer
Answer: carbon dioxide gas escapes
- The reaction makes carbon dioxide gas.
- The gas escapes from the open beaker, so its mass is not measured.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What does conservation of mass mean?
Show answer
The total mass of reactants equals the total mass of products.
Check 2
Why is mass conserved?
Show answer
Atoms are not made or destroyed, only rearranged.
Check 3
What is relative atomic mass?
Show answer
The mass of an atom of an element compared with carbon-12, shown on the periodic table.
Check 4
What is relative formula mass?
Show answer
The sum of the relative atomic masses of the atoms in a formula.
Check 5
What are the units of Mr?
Show answer
There are none.
Check 6
What is the Mr of H₂O?
Show answer
18.
Check 7
What is the Mr of CO₂?
Show answer
44.
Check 8
What is the Mr of NaOH?
Show answer
40.
Check 9
What is the Mr of CaCO₃?
Show answer
100.
Check 10
What is the Mr of H₂SO₄?
Show answer
98.
Check 11
Why does magnesium gain mass when burned in air?
Show answer
It combines with oxygen from the air.
Check 12
Why does a solid lose mass when it gives off a gas in an open container?
Show answer
The gas escapes and is not weighed.
Check 13
How do you find the mass of a gas that escaped?
Show answer
Subtract the final mass from the starting mass.
Check 14
What happens to total mass in a sealed container?
Show answer
It stays the same.
Check 15
How do you work out Mr with brackets?
Show answer
Multiply everything inside the bracket by the number outside.
Check 16
How can you check an equation is balanced using Mr?
Show answer
The total Mr of the reactants equals the total Mr of the products.
Check 17
What is the Mr of MgO?
Show answer
40.
Check 18
What is the Mr of NH₃?
Show answer
17.
Check 19
What is the Mr of HCl?
Show answer
36.5.
Check 20
What is the Mr of MgCl₂?
Show answer
95.
Check 21
What symbol is used for relative atomic mass?
Show answer
Ar.
Check 22
What symbol is used for relative formula mass?
Show answer
Mr.
Check 23
What Ar value is used for chlorine in calculations?
Show answer
35.5.
Check 24
What is a balanced equation?
Show answer
One with the same number of each type of atom on both sides.
Keep going with Relative formula mass and conservation of mass
Practise free
Yumio is not affiliated with, endorsed by or connected to AQA, Pearson Edexcel or any other exam board.