Grade: Grade 10 Subject: Science (Chemistry) Unit: Stoichiometry Lesson: 3 of 6 SAT: ProblemSolving+DataAnalysis ACT: Science

Guided Practice

Problem-Solving Strategy

Key conversion: Molar mass (g/mol), Avogadro's number (6.02 x 10^23), Molar volume at STP (22.4 L/mol)

Problem 1: How many moles are in 36 g of water (H2O)?

Molar mass of H2O = 18 g/mol. 36g / 18g/mol = 2 moles

Problem 2: How many grams are in 0.5 mol of NaCl?

Molar mass of NaCl = 58.5 g/mol. 0.5 mol x 58.5 g/mol = 29.25 g

Problem 3: For 2H2 + O2 → 2H2O, how many moles of water form from 3 mol H2?

Ratio is 2:2 (H2:H2O), so 3 mol H2 produces 3 mol H2O

Problem 4: How many particles are in 2 mol of any substance?

2 mol x 6.02 x 10^23 = 1.204 x 10^24 particles

Problem 5: What mass of O2 is needed to react with 4 mol of H2?

Ratio 2H2:1O2, so 4 mol H2 needs 2 mol O2. 2 mol x 32 g/mol = 64 g O2

Problem 6: How many moles in 44.8 L of gas at STP?

44.8 L / 22.4 L/mol = 2 moles

Problem 7: For N2 + 3H2 → 2NH3, how many moles of NH3 from 6 mol H2?

Ratio 3:2 (H2:NH3). 6 mol H2 x (2/3) = 4 mol NH3

Problem 8: What is the molar mass of CO2?

C = 12 + O2 = 32. Total = 44 g/mol

Problem 9: How many grams of CO2 form from burning 12 g of C?

C + O2 → CO2. 12g C = 1 mol C = 1 mol CO2 = 44 g CO2

Problem 10: How many molecules in 9 g of H2O?

9g / 18g/mol = 0.5 mol. 0.5 x 6.02 x 10^23 = 3.01 x 10^23 molecules