Stoichiometry MCQs
Inter · Chemistry (11th) · 6 questions
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11th class chemistry: Stoichiometry
Stoichiometry is counting particles by weighing them. One mole of anything contains Avogadro's number of particles, 6.022 × 10²³. The molar mass in grams equals the formula mass (H₂O = 18 g/mol, CO₂ = 44 g/mol), so moles = mass ÷ molar mass and particles = moles × 6.022 × 10²³. For gases at 273 K and 1 atm, one mole occupies 22.414 dm³. A balanced equation gives mole ratios, not mass ratios: in 2H₂ + O₂ → 2H₂O, two moles of hydrogen give two moles of water. To solve a problem, convert what you are given into moles, use the ratio from the equation, then convert back to grams, particles or volume. When two reactants are given, the limiting reactant is the one that runs out first; it decides the theoretical yield. Percentage yield = (actual yield ÷ theoretical yield) × 100.
Questions with answers
Written by Parcha from the syllabus. Samples until a subject reviewer checks them; report anything wrong from the practice screen.
Q1 · Parcha original · Sample
The value of Avogadro's number is:
- A. 6.022 × 10²³
- B. 6.022 × 10⁻²³
- C. 3.011 × 10²³
- D. 1.66 × 10⁻²⁴
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Answer: A. 6.022 × 10²³
One mole of any substance contains 6.022 × 10²³ particles (atoms, molecules or ions). 1.66 × 10⁻²⁴ g is one atomic mass unit.
Open this question on its own pageQ2 · Parcha original · Sample
How many moles are there in 36 g of water (H₂O, molar mass 18 g/mol)?
- A. 0.5
- B. 1
- C. 2
- D. 18
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Answer: C. 2
Moles = mass ÷ molar mass = 36 ÷ 18 = 2 mol.
Q3 · Parcha original · Sample
The mass of one mole of carbon dioxide (CO₂) is:
- A. 28 g
- B. 32 g
- C. 12 g
- D. 44 g
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Answer: D. 44 g
Molar mass of CO₂ = 12 + (2 × 16) = 44 g/mol, so one mole weighs 44 g.
Q4 · Parcha original · Sample
The limiting reactant in a reaction is the one that:
- A. Is present in the largest mass
- B. Is used up first and limits the amount of product
- C. Is left over at the end
- D. Acts as a catalyst
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Answer: B. Is used up first and limits the amount of product
The limiting reactant runs out first, so it decides how much product can form; the other reactant is left in excess. Compare mole ratios, not masses, to find it.
Q5 · Parcha original · Sample
How many molecules are there in 0.5 mol of a gas?
- A. 3.011 × 10²³
- B. 6.022 × 10²³
- C. 1.204 × 10²⁴
- D. 3.011 × 10²²
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Answer: A. 3.011 × 10²³
Number of molecules = moles × Avogadro's number = 0.5 × 6.022 × 10²³ = 3.011 × 10²³.
Q6 · Parcha original · Sample
For 2H₂ + O₂ → 2H₂O, how many moles of water form from 4 mol of H₂ with excess oxygen?
- A. 2
- B. 8
- C. 1
- D. 4
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Answer: D. 4
The equation gives 2 mol H₂ → 2 mol H₂O, a 1:1 ratio. So 4 mol H₂ gives 4 mol H₂O.