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Moles and stoichiometry

Written by Dr. Ayesha Nadeem (Chemistry)

MDCAT · Chemistry · 11 questions

Name: ____________________   Date: ____________

  1. How many moles are there in 36 g of water? (H = 1, O = 16)

    • A 3 moles
    • B 2 moles
    • C 1 mole
    • D 0.5 mole

  2. What is the mass of 0.5 mole of carbon dioxide? (C = 12, O = 16)

    • A 11 g
    • B 22 g
    • C 44 g
    • D 88 g

  3. How many molecules are there in 2 moles of a substance? (Avogadro's number is 6.02 x 10^23.)

    • A 3.01 x 10^23
    • B 1.204 x 10^24
    • C 6.02 x 10^23
    • D 1.204 x 10^23

  4. A compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Its empirical formula is:

    • A C2H2O
    • B CHO
    • C CH2O
    • D C2H4O2

  5. What volume do 2 moles of any ideal gas occupy at STP?

    • A 44.8 dm3
    • B 22.4 dm3
    • C 67.2 dm3
    • D 11.2 dm3

  6. In the reaction 2H2 + O2 -> 2H2O, how many moles of water form from 4 moles of H2 and 1 mole of O2?

    • A 1 mole
    • B 2 moles
    • C 3 moles
    • D 4 moles

  7. What is the molarity of a solution made by dissolving 4 g of NaOH in enough water to make 500 mL? (Na = 23, O = 16, H = 1)

    • A 0.8 M
    • B 0.2 M
    • C 0.1 M
    • D 0.4 M

  8. What is the percentage of oxygen by mass in water? (H = 1, O = 16)

    • A 50%
    • B 88.9%
    • C 94.4%
    • D 11.1%

  9. How many moles of oxygen atoms are in 1 mole of H₂SO₄?

    • A 1
    • B 6
    • C 2
    • D 4

  10. The empirical formula of a compound with the molecular formula C₆H₁₂O₆ is:

    • A C₂H₄O₂
    • B CH₂O
    • C C₃H₆O₃
    • D C₆H₁₂O₆

  11. What mass of calcium oxide forms when 100 g of calcium carbonate decomposes completely? (CaCO₃ = 100, CaO = 56)

    • A 56 g
    • B 44 g
    • C 28 g
    • D 100 g

Answer key

1. B, 2. B, 3. B, 4. C, 5. A, 6. B, 7. B, 8. B, 9. D, 10. B, 11. A

  1. The molar mass of water is 18 g/mol, so 36 g is 36 / 18 = 2 moles.

  2. The molar mass of CO2 is 44 g/mol, so 0.5 mole has a mass of 22 g.

  3. Number of molecules = 2 x 6.02 x 10^23 = 1.204 x 10^24.

  4. Moles: C = 40/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33. The ratio is 1 : 2 : 1, so the empirical formula is CH2O.

  5. One mole of an ideal gas occupies 22.4 dm3 at STP, so two moles occupy 44.8 dm3.

  6. One mole of O2 needs only 2 moles of H2. Oxygen is the limiting reagent and 1 mole of O2 gives 2 moles of water.

  7. 4 g / 40 g per mol = 0.1 mol. Dividing by 0.5 L gives 0.2 mol/L.

  8. Mass percentage = 16 / 18 x 100 = 88.9%.

  9. The formula has four oxygen atoms.

  10. Divide all subscripts by 6 to get the simplest ratio, CH₂O.

  11. CaCO₃ gives CaO and CO₂ in a 1 to 1 mole ratio. One mole (100 g) gives one mole (56 g) of CaO.