Common Mistakes
Top Mistakes to Avoid
Mistake #1: Using Unbalanced Equations
Error: Using mole ratios from an unbalanced equation.
Correct: Always balance the equation first!
Mistake #2: Wrong Molar Mass
Error: Forgetting to multiply by subscripts (H2O = 1+16 = 17).
Correct: H2O = 2(1) + 16 = 18 g/mol
Mistake #3: Assuming More = Limiting
Error: Thinking the reactant with smaller amount is always limiting.
Correct: Compare using mole ratios, not just amounts.
Mistake #4: Inverting Mole Ratios
Error: Using wrong ratio direction in calculations.
Correct: Set up ratios so units cancel properly.
Practice: Find the Errors
Problem 1: Student calculates molar mass of H2SO4 as 1+32+64 = 97. Correct?
Error: Should be 2(1)+32+4(16) = 2+32+64 = 98 g/mol
Problem 2: Student uses H2 + O2 → H2O to find moles without balancing. Error?
Error: Equation must be balanced: 2H2 + O2 → 2H2O
Problem 3: With 3 mol A and 2 mol B, student says B is limiting because 2 < 3. Always true?
Error: Must consider mole ratio. If ratio is 1A:2B, then A might be limiting.
Problem 4: Student divides by 6.02 x 10^23 to convert moles to particles. Correct?
Error: Should MULTIPLY moles by Avogadro's number to get particles.
Problem 5: Student gets percent yield of 110%. Possible?
Error: Percent yield cannot exceed 100%. Check calculations for errors.
Problem 6: Student uses 22.4 L/mol for a gas at 100C. Correct?
Error: 22.4 L/mol only applies at STP (0C, 1 atm). Higher T means larger volume.
Problem 7: For 2A + B → C, student says 2 mol A and 2 mol B gives 2 mol C. Correct?
Error: B is limiting. 2 mol B uses 4 mol A (only have 2). So A is actually limiting: 2 mol A → 1 mol C.
Problem 8: Student forgets to convert grams to moles before using mole ratio. Error?
Error: Yes! Must convert to moles first, then use mole ratio, then convert back if needed.
Problem 9: Student says actual yield is always less than theoretical. True?
Correct! Due to incomplete reactions, side reactions, or loss during processing, actual yield <= theoretical.
Problem 10: Molar mass of NaCl calculated as 23+17 = 40. Correct?
Error: Cl is 35.5, not 17. NaCl = 23 + 35.5 = 58.5 g/mol