DAT General Chemistry: Stoichiometry 2 — Questions and Answers
Question 1: How many moles of oxygen gas are required to completely combust 2 moles of propane (C3H8)?
- 5 mol
- 10 mol (Correct answer)
- 3 mol
- 8 mol
Correct answer: 10 mol
Balanced equation C3H8 + 5O2 → 3CO2 + 4H2O needs 5 mol O2 per mol propane, so 2 mol requires 10 mol.
Question 2: What is the percent composition by mass of carbon in CO2 (C=12, O=16)?
- 27.3% (Correct answer)
- 50.0%
- 72.7%
- 12.0%
Correct answer: 27.3%
Carbon's 12 g out of CO2's molar mass of 44 g gives 12/44 = 27.3%.
Question 3: If 3 mol of N2 react with 6 mol of H2, which is the limiting reagent in N2 + 3H2 → 2NH3?
- N2
- H2 (Correct answer)
- Neither, exactly stoichiometric
- NH3
Correct answer: H2
3 mol N2 needs 9 mol H2 but only 6 mol is present, so H2 limits the reaction.
Question 4: How many grams are in 0.50 mol of NaCl (Na=23, Cl=35.5)?
- 29.25 g (Correct answer)
- 58.5 g
- 14.6 g
- 117 g
Correct answer: 29.25 g
NaCl molar mass is 58.5 g/mol, so 0.50 mol equals 29.25 g.
Question 5: What is the empirical formula of a compound that is 40% C, 6.7% H, and 53.3% O by mass?
- CHO
- CH2O (Correct answer)
- C2H4O2
- CH4O
Correct answer: CH2O
Dividing by atomic masses gives a 1:2:1 mole ratio of C:H:O, the formula CH2O.
Question 6: How many molecules are in 2.0 mol of water?
- 6.02 × 10^23
- 1.2 × 10^24 (Correct answer)
- 3.01 × 10^23
- 1.2 × 10^23
Correct answer: 1.2 × 10^24
Multiplying 2.0 mol by Avogadro's number gives 1.2 × 10^24 molecules.
Question 7: In the reaction 2H2 + O2 → 2H2O, how many grams of water form from 4 g of H2 (excess O2, H=1, O=16)?
- 18 g
- 36 g (Correct answer)
- 72 g
- 9 g
Correct answer: 36 g
4 g H2 is 2 mol, producing 2 mol water at 18 g/mol = 36 g.
DAT General Chemistry: Stoichiometry 2
How many moles of oxygen gas are required to completely combust 2 moles of propane (C3H8)?