Stoichiometry — Drills · Grade 11 Science

Chapter: Stoichiometry & Yield

What these drills covers

By the end of Stoichiometry you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: stoichiometry, mole ratio, mass-to-mass calculation, given, unknown.

Problem 1

The coefficients of a balanced equation give a ratio between substances measured in…
  1. atoms per gram
  2. grams
  3. moles
  4. milliliters
Answer and explanation

moles

Coefficients give mole ratios, which is why grams must be converted to moles before they can be used.

Problem 2

For N₂ + 3H₂ → 2NH₃, how many moles of H₂ are needed to react completely with 2.0 mol of N₂?
Answer and explanation

6 mol

The mole ratio N₂ : H₂ is 1 : 3, so 2.0 mol N₂ × 3 = 6.0 mol H₂.

Problem 3

For 2H₂ + O₂ → 2H₂O, how many moles of water form from 5.0 mol of O₂ reacting with excess H₂?
Answer and explanation

10 mol

The mole ratio O₂ : H₂O is 1 : 2, so 5.0 mol O₂ × 2 = 10.0 mol H₂O.

Problem 4

For C + O₂ → CO₂, what mass of carbon dioxide forms from 2.00 mol of carbon? (Molar mass CO₂ = 44.01 g/mol.)
Answer and explanation

88.02 g

Ratio C : CO₂ = 1 : 1, so 2.00 mol C → 2.00 mol CO₂; 2.00 mol × 44.01 g/mol = 88.02 g.

Problem 5

Why must you convert a mass in grams to moles before using the coefficients of a balanced equation?
Answer and explanation

The coefficients are a ratio of moles, not of grams, because equal masses of different substances contain different numbers of particles. Converting to moles first lets you correctly apply the mole ratio, then you can convert back to grams at the end.

Problem 6

For N₂ + 3H₂ → 2NH₃, what mass of ammonia forms from 28.02 g of N₂ with excess H₂? (N₂ = 28.02 g/mol, NH₃ = 17.04 g/mol.)
Answer and explanation

34.08 g

28.02 g ÷ 28.02 g/mol = 1.00 mol N₂; ratio NH₃ : N₂ = 2 : 1 gives 2.00 mol NH₃; 2.00 mol × 17.04 g/mol = 34.08 g.

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