Unit 9 Stoicheometry

How many moles are in 36.0 g of water (H₂O)?

Answer explanation

Molar mass of water is 18.0 g/mol, so 36.0 ÷ 18.0 = 2.00 mol.

Which of the following represents a mole ratio from the equation: 2H₂ + O₂ → 2H₂O?

• 2 mol H₂ : 2 mol H₂O
• 1 mol H₂ : 1 mol O₂
• 2 mol O₂ : 2 mol H₂
• 2 mol H₂O : 1 mol H₂
• 1 mol H₂O : 2 mol O₂

Answer explanation

From the balanced equation, 2 mol H₂ yields 2 mol H₂O.

Which unit is commonly used to express molar mass?

• g/mL
• g/mol
• mol/L
• mol/m³
• atoms/mol

Answer explanation

Molar mass is expressed in grams per mole (g/mol).

You have 6.02 × 10²³ molecules of CO₂. How many moles is this?

Answer explanation

6.02 × 10²³ particles = 1 mol (Avogadro's number).

From the equation: N₂ + 3H₂ → 2NH₃, what is the mole ratio of H₂ to NH₃?

• 1:1
• 3:2
• 2:3
• 3:1
• 2:1

Answer explanation

3 mol H₂ react to form 2 mol NH₃, ratio is 3:2.

Select all correct mole-to-mass conversions.

• 2 mol Na = 46.0 g Na
• 1 mol H₂O = 18.0 g
• 0.5 mol CO₂ = 22.0 g
• 3 mol O₂ = 96.0 g
• 1 mol C = 6.02 × 10²³ g

Answer explanation

CO₂ molar mass = 44 g/mol, so 0.5 mol = 22 g. C is 12 g/mol, not 6.02×10²³ g.

Challenge: How many molecules are in 36.0 g of water (H₂O)?

Answer explanation

36 g ÷ 18 g/mol = 2 mol; 2 × 6.02 × 10²³ = 1.204 × 10²⁴ molecules.

What step comes first in a stoichiometry problem?

• Convert to moles
• Check sig figs
• Write and balance the chemical equation
• Calculate percent yield
• Convert to desired unit

Answer explanation

Stoichiometry always starts with a balanced equation.

Challenge: If 4.0 mol H₂ reacts completely with excess O₂, how many grams of H₂O are produced?

Answer explanation

4 mol H₂ → 4 mol H₂O × 18 g/mol = 72.0 g H₂O.

Why do you need a balanced equation before doing mole ratio calculations?

• It shows the limiting reactant
• It ensures unit cancellation
• It confirms Avogadro’s number
• It provides correct mole relationships
• It allows molarity calculations

Answer explanation

Balanced equations give the mole ratio between reactants and products.