Unit 3 · 11 lessons

Quantities in Chemical Reactions

The mole lets chemists count invisibly small particles by weighing them on a scale, and stoichiometry uses that idea to predict exactly how much of each substance reacts or forms.

121 practice questions
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Lessons

1Significant Figures & Unit AnalysisRules for counting and rounding significant digits, applying them correctly through calculations, and using dimensional analysis to convert between units safely.11 questions
2The Mole ConceptWhy chemists count atoms and molecules in groups of $6.022 \times 1 0^{23}$ — the mole is a counting unit exactly like a dozen, just scaled up to handle unimaginably small, unimaginably numerous particles.11 questions
3Molar MassCalculating the molar mass ( $M$ , in g/mol) of elements and compounds by summing atomic masses from the periodic table — the bridge between a mass you can weigh and a mole count you can use.11 questions
4Mass, Moles & Number of ParticlesConverting between mass, moles, and number of particles using the two big conversion factors — molar mass and Avogadro's number — often drawn as a "mole highway" or "mole map."11 questions
5Percentage CompositionCalculating what percentage of a compound's mass comes from each element it contains — the first step toward determining an unknown compound's formula from experimental data.11 questions
6Empirical & Molecular FormulasWorking backward from percent composition (or mass data) to a compound's simplest whole-number formula, and from there to its true molecular formula when the actual molar mass is known.11 questions
7HydratesFormulas of hydrates like $CuSO_{4} \cdot 5 H_{2} O$ , and finding the number of water molecules per formula unit from mass-loss-on-heating data.11 questions
8Mole Ratios from Balanced EquationsReading mole ratios directly from the coefficients of a balanced chemical equation, and using them to move from moles of one substance to moles of another — the pivot step of every stoichiometry problem.11 questions
9Mass-to-Mass StoichiometryCalculating the mass of a reactant or product from the mass of another reactant or product, by chaining mole conversions on either side of a mole-ratio step.11 questions
10Limiting & Excess ReagentsIdentifying which reactant runs out first in a real reaction (the limiting reagent), and calculating how much of the other reactant (the excess reagent) is left over unreacted.11 questions
11Percentage YieldComparing the theoretical yield predicted by stoichiometry to the actual yield measured in the lab, and understanding why real reactions almost always produce less than the calculation predicts.11 questions