L31. Chemical Quantities

Stoichiometry and Energy

R-report

L31. Chemical Quantities

How can the numbers in a balanced equation tell you exactly how many moles of each substance are needed?

Central Idea:

Chemical equations tell a story about amounts: balanced coefficients give the mole ratios between reactants and products so you can predict how much of each substance participates. For example, the balanced equation 2H2 + O2 → 2H2O means two moles of hydrogen gas react with one mole of oxygen gas to produce two moles of water. Coefficients compare moles directly; they do not automatically compare grams, molecules, or volumes unless you convert units first. Always read coefficients as “moles of” when moving from an equation to a calculation. This core idea lets us use the same ratio to safely scale up from lab samples to industry while keeping atoms conserved.

How To Use Coefficients:

- Identify the balanced equation first (example: 2H2 + O2 → 2H2O); read coefficients as moles and test by counting atoms on each side. - Choose the mole ratio you need (example: to find O2 from H2 use 2 mol H2 : 1 mol O2); separate observation from reasoning and try the reverse ratio. - Convert given amounts into moles when needed (example: 4.04 g H2 → 2.00 mol H2 using 2.02 g·mol−1); include units and show work. - Multiply the known moles by the coefficient ratio (example: 3.0 mol H2 × (1 mol O2 / 2 mol H2) = 1.5 mol O2); then verify by converting to grams, checking atoms, and confirming units.

Apply Mole Ratios:

Compare what stays the same and what changes: atoms of each element must be conserved, while amounts in moles change according to coefficients. Ask the key question: do the coefficients in the balanced equation give a direct mole-to-mole conversion between the substances you have and the substance you want? Test this by attempting the reversal: use the equation to predict backwards and check that numbers invert correctly. Look for evidence of a chemical change (gas release, precipitate, temperature change) rather than a simple physical change; if evidence disagrees, recheck your equation or limiting reagent. Try simple tests: measure gas, filter solids, or weigh materials.

Synthesize Ideas:

Restate: Balanced coefficients provide mole ratios for calculations (for example 2H2 + O2 → 2H2O gives 2 mol H2 : 1 mol O2 : 2 mol H2O). Key vocabulary: coefficient, mole, mole ratio, limiting reagent. Example equation: 4.04 g H2 → 2.00 mol H2; then use 2 mol H2 / 1 mol O2 to find O2. Acceptance checklist: include units, show mole conversions, and check atom balance, and label steps.

Summary action: Use the balanced equation to set up mole ratios before converting units. Vocabulary to use: molar mass, coefficient ratio, product, reactant. Example calculation: 3.0 mol H2 × (1 mol O2 / 2 mol H2) = 1.5 mol O2; convert to grams if needed. Final checklist: verify units cancel, confirm atom-level story (H and O counts), and match predicted evidence like gas formation or mass change and show work.