Worked example
For 2H₂ + O₂ → 2H₂O, what amount of O₂ is required for 4 mol H₂, assuming the reaction follows this ratio?
Answer & why
Answer: 2 mol
The H₂:O₂ ratio is 2:1, so 4 mol H₂ requires 2 mol O₂.
Grade 10 · Science
Use ratios in balanced reactions
Use a supplied balanced equation or molar mass to calculate amounts with stated units.
6 authored questions + worked example
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All questions
All 6 authored questions for this skill.
Question 1
A substance has molar mass 18 g/mol. What mass is 3 mol?
- 18 g
- 54 g
- 21 g
- 6 g
A hint
Read coefficients as mole ratios, and use only the supplied molar masses for mass conversions.
Answer & why
Answer: 54 g
3 mol × 18 g/mol = 54 g.
Question 2
A sample has mass 44 g and molar mass 22 g/mol. How many moles are present?
- 968 mol
- 22 mol
- 2 mol
- 66 mol
A hint
Read coefficients as mole ratios, and use only the supplied molar masses for mass conversions.
Answer & why
Answer: 2 mol
44 g ÷ 22 g/mol = 2 mol.
Question 3
The stated reaction ratio uses 2 mol of substance A plus 1 mol B to produce 1 mol C. Starting with 4 mol A and 1 mol B, what is the maximum C?
- 1 mol
- 5 mol
- 4 mol
- 2 mol
A hint
Read coefficients as mole ratios, and use only the supplied molar masses for mass conversions.
Answer & why
Answer: 1 mol
The 1 mol B supports one reaction ratio, consuming 2 mol A and producing 1 mol C.
Question 4
An equation's coefficients are 2:1 for substances A:B. Does that alone mean the required masses in grams are 2:1?
- No; masses also depend on each substance's molar mass.
- Yes; coefficients always give equal gram ratios.
- No; coefficients cannot describe mole ratios.
- Yes; every substance has the same molar mass.
A hint
Read coefficients as mole ratios, and use only the supplied molar masses for mass conversions.
Answer & why
Answer: No; masses also depend on each substance's molar mass.
Coefficients give amount ratios; different molar masses change the mass ratio.
Question 5
For 2H₂ + O₂ → 2H₂O, what amount of O₂ is required for 4 mol H₂, assuming the reaction follows this ratio?
- 2 mol
- 8 mol
- 4 mol
- 1 mol
A hint
Read coefficients as mole ratios, and use only the supplied molar masses for mass conversions.
Answer & why
Answer: 2 mol
The H₂:O₂ ratio is 2:1, so 4 mol H₂ requires 2 mol O₂.
Question 6
For N₂ + 3H₂ → 2NH₃, how many moles of NH₃ correspond to 2 mol N₂ when enough H₂ is available?
- 6 mol
- 1 mol
- 4 mol
- 2 mol
A hint
Read coefficients as mole ratios, and use only the supplied molar masses for mass conversions.
Answer & why
Answer: 4 mol
Each mole of N₂ corresponds to 2 mol NH₃; doubling gives 4 mol.