Stoichiometry Mastery Study Guide
Stoichiometry is the accounting department of chemistry: given a balanced equation, it tells you exactly how much reactant you need and how much product you can get. Almost every later topic — gas volumes, solution concentrations, equilibrium, thermochemistry — runs on these same conversion habits, so this guide drills the full pipeline from grams to moles to mole ratios to answers, plus the limiting-reactant and percent-yield thinking that real lab work adds.
Key formulas
n = m / M, where n is moles, m is mass in grams, and M is molar mass in g/mol
N = n x N_A, where N is particle count and N_A = 6.022 x 10^23 particles/mol
Percent composition of an element = (total mass of that element in one mole of compound / molar mass of compound) x 100%
M1 x V1 = M2 x V2 for dilutions, where M is molarity (mol/L) and V is volume
Percent yield = (actual yield / theoretical yield) x 100%
Empirical formula: mass percent -> grams -> moles of each element -> divide by smallest -> whole-number ratio
Mole ratio = coefficient of desired substance / coefficient of given substance, taken from the balanced equation
Topic checklist
The mole concept
The mole is the chemist's counting unit: 1 mol = 6.022 x 10^23 particles (atoms, molecules, or formula units).
Avogadro's number converts between moles and particle counts: particles = n x N_A.
Use dimensional analysis so every conversion cancels its units explicitly.
Molar mass
Molar mass (g/mol) is the bridge between grams on the balance and moles in the equation.
For diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2), multiply the atomic mass by 2.
Round atomic masses from the periodic table but keep enough digits to match the problem's precision.
Balancing equations first
Every stoichiometry calculation starts from a BALANCED equation — mole ratios come only from its coefficients.
Subscripts describe a compound's fixed composition and are never changed when balancing.
The standard road map: given units -> moles -> mole ratio -> desired units.
Limiting reactant
Identify the limiting reactant by converting each reactant's mass to moles of product, or compare mole ratios directly.
Excess reactants are left over; the limiting reactant determines the maximum product.
Excess mass can be found by subtracting the consumed amount from the starting amount.
Percent yield
Percent yield = (actual yield / theoretical yield) x 100%; it can never exceed 100%.
Theoretical yield comes from the limiting reactant; actual yield comes from the lab.
Common causes of <100% yield: incomplete reaction, side reactions, and product lost in purification.
Percent composition and formulas
Percent composition = (total mass of the element in the formula / molar mass of the compound) x 100%.
Empirical formulas come from mass percent: convert % to grams, grams to moles, then find the simplest whole-number ratio.
Molecular formula = empirical formula scaled by the true molar mass.
Common mistakes
Mistake: Changing subscripts instead of coefficients when balancing equations
Fix:
Changing a subscript creates a different compound; changing a coefficient only changes how many molecules react. Rewrite the compound if it is wrong, then balance with coefficients only.
Mistake: Skipping the balancing step and using coefficients from an unbalanced equation
Fix:
Step one is always: write the equation and balance it before any mole ratio is used. A ratio taken from an unbalanced equation is meaningless.
Mistake: Using atomic mass when the substance is diatomic, e.g. treating O2 as 16.00 g/mol
Fix:
Look up the actual molecular form. Water is H2O (18.02 g/mol), not O (16.00); oxygen gas is O2 (32.00 g/mol), not O. The substance named decides the formula used.
Mistake: Reporting a percent yield above 100% without recognizing it as impossible
Fix:
Recheck the calculation — you likely used the wrong reactant as limiting or divided by the wrong molar mass. Percent yield above 100% means an arithmetic or measurement error, not a great reaction.
Exam tips
For mixed practice, work through stoichiometry-practice end to end before the exam — it chains conversions the way real problems do.
If conversions feel shaky, go back to mole-concept-practice and drill grams-to-moles-to-particles in both directions.
Slow on equations? Run balancing-equations-practice timed until you can balance five in a row without notes.
On exam day, write the road map (given -> moles -> ratio -> answer) on scratch paper first; it prevents skipped steps.
Always check units at the end: grams of the requested substance, not grams of what you started with.