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Moles and stoichiometry
8 MDCAT Chemistry questions with answers and explanations.
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How many moles are there in 36 g of water? (H = 1, O = 16)
- 1 1 mole
- 2 2 moles
- 3 3 moles
- 4 0.5 mole
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Correct answer: 2 moles
The molar mass of water is 18 g/mol, so 36 g is 36 / 18 = 2 moles.
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What is the mass of 0.5 mole of carbon dioxide? (C = 12, O = 16)
- 1 44 g
- 2 11 g
- 3 22 g
- 4 88 g
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Correct answer: 22 g
The molar mass of CO2 is 44 g/mol, so 0.5 mole has a mass of 22 g.
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How many molecules are there in 2 moles of a substance? (Avogadro's number is 6.02 x 10^23.)
- 1 6.02 x 10^23
- 2 3.01 x 10^23
- 3 1.204 x 10^24
- 4 1.204 x 10^23
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Correct answer: 1.204 x 10^24
Number of molecules = 2 x 6.02 x 10^23 = 1.204 x 10^24.
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A compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Its empirical formula is:
- 1 CH2O
- 2 C2H4O2
- 3 CHO
- 4 C2H2O
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Correct answer: CH2O
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.
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What volume do 2 moles of any ideal gas occupy at STP?
- 1 22.4 dm3
- 2 11.2 dm3
- 3 44.8 dm3
- 4 67.2 dm3
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Correct answer: 44.8 dm3
One mole of an ideal gas occupies 22.4 dm3 at STP, so two moles occupy 44.8 dm3.
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In the reaction 2H2 + O2 -> 2H2O, how many moles of water form from 4 moles of H2 and 1 mole of O2?
- 1 4 moles
- 2 2 moles
- 3 1 mole
- 4 3 moles
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Correct answer: 2 moles
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.
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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)
- 1 0.1 M
- 2 0.2 M
- 3 0.4 M
- 4 0.8 M
Show answer
Correct answer: 0.2 M
4 g / 40 g per mol = 0.1 mol. Dividing by 0.5 L gives 0.2 mol/L.
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What is the percentage of oxygen by mass in water? (H = 1, O = 16)
- 1 11.1%
- 2 50%
- 3 88.9%
- 4 94.4%