Stoichiometry and Chemical Calculations
What you must understand
Master the mole concept: Avogadro's number, molar mass, and the conversion chain between mass, moles, and particles. Balance equations and use mole ratios to solve limiting-reactant and percent-yield problems. Handle solution chemistry with molarity, dilution, and titration calculations. Know empirical versus molecular formulas and how combustion data yield them. Understand gas stoichiometry at STP and via the ideal gas law, and be comfortable with significant figures and unit cancellation.
Traps to watch for
- Skipping the limiting-reactant check and using the more abundant reagent.
- Confusing percent yield with percent error, or applying yield before determining the limiting reagent.
- Using the volume of solvent instead of the volume of solution in molarity.
- Forgetting to convert grams to moles before applying mole ratios.
- Rounding intermediate values too early and losing precision.
Worked example
How many grams of water form when 4.0 g of H2 reacts with 32.0 g of O2? Moles of H2 equal 4.0/2.02, about 1.98 mol; moles of O2 equal 32.0/32.00, exactly 1.00 mol. The equation 2H2 + O2 gives 2H2O needs two moles of H2 per mole of O2, so H2 is limiting. Water formed equals 1.98 mol, and its mass is 1.98 times 18.02, about 35.7 g. Oxygen is in excess.
Rule of thumb
Always convert to moles, divide each reactant's moles by its coefficient, and the smallest quotient identifies the limiting reactant.