Chapter 8 · Quantities in Chemical Reactions
8.4Making Molecules: Mass-to-Mass Conversions
12 min · two checks
Predict
What is the full map from grams of reactant to grams of product?
The idea
- Carry out a mass-to-mass stoichiometry problem.
- Refuse to multiply grams by a coefficient directly.
Mass-to-mass problems feel long and are three short conversions. Convert the given mass to moles. Use the mole ratio. Convert the wanted moles to mass. For “grams of CO₂ from 16.0 g of CH₄,” methane’s molar mass is 16.04 g/mol, so you have about 1.00 mol CH₄, which makes 1.00 mol CO₂, which has a mass of 44.0 g. The grams are not in a 1:1 ratio even though the moles are, because the molar masses differ.
Significant figures come from the measured mass and any molar masses you were told to limit. Coefficients are exact. If the path produces an absurd mass — more product grams than the reactants could supply by conservation of mass — a factor is upside down or oxygen’s mass was forgotten. Products cannot outweigh the reactants that formed them.
Keep these
- g → mol → mol → g.
- Each substance uses its own molar mass.
- Product mass cannot exceed total reactant mass.
Worked path
What mass of water forms when 8.00 g of O₂ reacts with excess H₂? 2 H₂ + O₂ → 2 H₂O. Molar masses: O₂ 32.00, H₂O 18.02.
Observe
8.00 g O₂ is the given. H₂ is excess, so oxygen decides the yield.