Stoichiometry Cheat Sheet
Mole ratios, mass-to-mass, limiting reactant, excess, and percent yield.
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Stoichiometry
Mole ratios, mass-to-mass, limiting reactant, excess, and percent yield.
Balance first
CORE RULEBalance the equation before any calculation. The coefficients are the source of every mole ratio you will use.
Mole ratio (stoichiometric factor)
CORE RULECoefficients give the conversion factor between substances. The factor from A to B is (coefficient of B) / (coefficient of A).
Convert to moles first
CORE RULEAnything not in moles must become moles before the ratio: mass ÷ molar mass; gas at STP ÷ 22.4 L/mol; particle count ÷ Nₐ.
Mass-to-mass (four steps)
METHODmass A → mol A (÷ MM_A) → mol B (× stoichiometric factor) → mass B (× MM_B). Carry units so they cancel.
Find the limiting reactant
METHODWhen both reactant amounts are given: convert each to moles, divide each by its coefficient; the smaller quotient is the limiting reactant. (Or compute product from each; the smaller wins.)
Use the limiting reactant
CORE RULECompute product from the limiting reactant's moles only (never the excess): × stoichiometric factor × product molar mass.
Excess reactant remaining
CORE RULEConsumed excess = mol LR × (excess coeff / LR coeff) × MM_excess. Remaining = initial excess − consumed. A negative result means the LR was misidentified.
Percent yield
CORE RULEPercent yield = (actual / theoretical) × 100. Theoretical comes from the limiting reactant at 100% conversion.
Theoretical from a known yield
CORE RULEGiven actual amount and percent yield: theoretical = (actual × 100) / percent yield.
Reverse-yield (work backwards)
METHODGiven actual product + percent yield, find reactant needed: actual → theoretical (via % yield) → mol product → mol reactant (ratio) → mass reactant (× MM).
Gas at STP
CORE RULEAt STP (273.15 K, 1 atm) one mole of any ideal gas occupies 22.4 L. Use 22.4 L/mol to convert moles ↔ volume for gas-phase species only (not liquids/solids).
Mass-balance check
WATCH OUTConservation of mass: total mass of reactants consumed = total mass of products formed. Use it as an independent check on the answer.