Chemistry Clarity · chemistryclarity.com

Gas Mixtures and Gas Stoichiometry: Quiz

Seven questions on partial pressures, mole fractions, gases collected over water and gas stoichiometry, with explanations.

7 questionsIntermediate

7 questions

  1. Question 1EasyA mixture contains N₂ at 0.60 atm, O₂ at 0.25 atm and CO₂ at 0.15 atm. What is the total pressure?
    Show answer

    Answer: 1.00 atm

    Dalton's law: P(total) = 0.60 atm + 0.25 atm + 0.15 atm = 1.00 atm.

  2. Question 2EasyA gas mixture is 40 % He by moles at a total pressure of 2.0 atm. What is the partial pressure of He?
    Show answer

    Answer: 0.80 atm

    P(He) = x(He) × P(total) = 0.40 × 2.0 atm = 0.80 atm.

  3. Question 3EasyOxygen is collected over water at 25 °C (water vapour pressure 23.8 mmHg). The total pressure is 760 mmHg. What is the pressure of the oxygen?
    Show answer

    Answer: 736.2 mmHg

    P(O₂) = P(total) − P(H₂O) = 760 mmHg − 23.8 mmHg = 736.2 mmHg.

  4. Question 4MediumWhat volume does 0.500 mol of a gas occupy at 0 °C and 1 atm?
    Show answer

    Answer: 11.2 L

    0.500 mol × 22.4 L/mol = 11.2 L. 12.2 L would be the volume at 25 °C (24.5 L/mol).

  5. Question 5MediumAt the same temperature and pressure, what volume of NH₃ forms from 10.0 L of N₂? (N₂ + 3H₂ → 2NH₃)
    Show answer

    Answer: 20.0 L

    For gases at the same T and P, volumes are in the mole ratio: 1 N₂ : 2 NH₃, so 10.0 L × 2 = 20.0 L.

  6. Question 6HardWhat volume of H₂ at 25 °C and 1.00 atm is produced when 1.31 g of Zn (65.38 g/mol) reacts with excess HCl? (Zn + 2HCl → ZnCl₂ + H₂)
    Show answer

    Answer: 0.490 L

    n(Zn) = 1.31 g ÷ 65.38 g/mol = 0.0200 mol = n(H₂). V = nRT/P = 0.0200 mol × 0.08206 L·atm/(mol·K) × 298.15 K ÷ 1.00 atm = 0.490 L. 0.449 L uses 22.4 L/mol at the wrong temperature.

  7. Question 7MediumWhy must the vapour pressure of water be subtracted when a gas is collected over water?
    Show answer

    Answer: The collected gas is mixed with water vapour, which contributes to the total pressure

    By Dalton's law the measured pressure is the sum of the gas pressure and the water vapour pressure, so the vapour pressure is subtracted.