AP Chemistry: Chemical Reactions & Stoichiometry 2 — Questions and Answers
Question 1: How many moles of O2 are required to completely combust 2 mol of C3H8 (propane)?
- 5 mol
- 10 mol (Correct answer)
- 3 mol
- 7 mol
Correct answer: 10 mol
Balanced equation C3H8 + 5O2 -> 3CO2 + 4H2O needs 5 mol O2 per mol propane, so 2 mol needs 10 mol.
Question 2: What is the molar mass of calcium phosphate, Ca3(PO4)2?
- 310.18 g/mol (Correct answer)
- 215.21 g/mol
- 278.18 g/mol
- 190.30 g/mol
Correct answer: 310.18 g/mol
3(40.08) + 2(30.97) + 8(16.00) = 310.18 g/mol.
Question 3: In the reaction 2Al + 3Cl2 -> 2AlCl3, if 4 mol Al react, how many mol AlCl3 form?
- 2 mol
- 3 mol
- 4 mol (Correct answer)
- 6 mol
Correct answer: 4 mol
The Al:AlCl3 ratio is 2:2 (1:1), so 4 mol Al yields 4 mol AlCl3.
Question 4: Which quantity equals one mole of any substance?
- 6.022 x 10^23 representative particles (Correct answer)
- 1 gram of the substance
- 22.4 grams at STP
- The atomic number in grams
Correct answer: 6.022 x 10^23 representative particles
A mole is defined as 6.022 x 10^23 (Avogadro's number) of representative particles.
Question 5: What is the percent by mass of nitrogen in NH4NO3?
- 17.5%
- 35.0% (Correct answer)
- 23.3%
- 46.7%
Correct answer: 35.0%
Two N atoms (28.02 g) over molar mass 80.06 g gives about 35.0%.
Question 6: A reaction produces 18.0 g of water from an actual yield experiment whose theoretical yield is 24.0 g. What is the percent yield?
- 133%
- 75.0% (Correct answer)
- 60.0%
- 42.0%
Correct answer: 75.0%
Percent yield = (18.0/24.0) x 100 = 75.0%.
Question 7: How many atoms of oxygen are present in 0.500 mol of CO2?
- 3.01 x 10^23
- 6.02 x 10^23 (Correct answer)
- 1.51 x 10^23
- 1.20 x 10^24
Correct answer: 6.02 x 10^23
0.500 mol CO2 has 1.00 mol O atoms = 6.02 x 10^23 atoms.
AP Chemistry: Chemical Reactions & Stoichiometry 2
How many moles of O2 are required to completely combust 2 mol of C3H8 (propane)?