Summary of "Stoichiometry Tutorial: Step by Step Video + review problems explained | Crash Chemistry Academy"

Concise summary — main ideas, concepts, methods, examples

Stoichiometry relates the amounts of reactants and products in a balanced chemical equation using the coefficients.
Coefficients represent fixed ratios (they can mean numbers of particles or moles) that remain constant regardless of reaction scale.
The central stoichiometric operation is a mole-to-mole conversion using the coefficient ratio from the balanced equation.
Real problems often require converting between mass, moles, particles, or gas volume; these are handled by chaining conversions (dimensional analysis).
Important constants/conversions:
- Molar mass: grams per mole (g/mol)
- Avogadro’s number: 6.02 × 10^23 particles per mole
- At STP: 1 mol gas ≈ 22.4 L

Follow these steps in order when solving stoichiometry problems:

Balance the chemical equation. The balanced coefficients provide the mole ratios used in conversions.
Identify what is given (A) and what is wanted (B).
Convert the given quantity to moles if it is not already in moles:
- If given mass → use molar mass (g → mol).
- If given particles (molecules/atoms/formula units) → use Avogadro’s number (particles → mol).
- If given gas volume at STP → use 22.4 L per mole (L → mol).
Perform the mole-to-mole conversion using the balanced equation’s coefficients:
- Use a fraction (moles of B / moles of A) so that the “A” unit (what you start with) is on the bottom (cancels) and “B” (what you want) is on top.
Convert moles of the desired substance to the required final units:
- mol → mass (use molar mass), mol → particles (Avogadro), mol → volume at STP (22.4 L/mol), etc.
Set up the calculation as a unit-cancelling chain (dimensional analysis). Check that only the wanted units remain. If units don’t cancel correctly, the setup is wrong.
Calculate numerically (multiply all numerators and divide by all denominators). Report the answer with appropriate significant figures if required.

Tip: Always set up conversions so the given units cancel out step by step — that makes errors easy to spot.

Example equation: H2 + N2 → NH3
Balanced: 3 H2 + 1 N2 → 2 NH3
This demonstrates the constant 3:1:2 ratio (H2 : N2 : NH3) which applies at the particle level or molar level.

Given 7.5 mol H2 → mol N2:
- Use ratio (1 mol N2 / 3 mol H2): 7.5 × (1/3) = 2.5 mol N2
Given 0.8 mol NH3 → mol H2:
- Use ratio (3 mol H2 / 2 mol NH3): 0.8 × (3/2) = 1.2 mol H2

Given 42.0 g N2 → mass NH3
1. 42.0 g N2 × (1 mol N2 / 28 g N2) = 1.5 mol N2
2. 1.5 mol N2 × (2 mol NH3 / 1 mol N2) = 3.0 mol NH3
3. 3.0 mol NH3 × (17 g NH3 / 1 mol NH3) = 51 g NH3
Final: 42.0 g N2 produces 51 g NH3 (assuming complete reaction).

Given 5.95 g NH3 → mass O2 required
- Map: g NH3 → mol NH3 → mol O2 via coefficients → g O2
- Result cited: 19.6 g O2 required (using the balanced equation’s coefficients and molar masses).

Balanced, then mass chain:
- Given 37.5 g C2H6 → 110 g CO2 produced (mass → mol → mole ratio → mass).

Given 2.8 × 10^24 molecules of C2H6 → convert molecules → mol C2H6 → mol H2O via coefficients → mass H2O
- Result cited: 251 g H2O produced.

Always balance the equation first.
Use mole ratios (from coefficients) for mole-to-mole conversions.
Place the given unit where it cancels (given on the bottom of the conversion fraction).
Confirm units cancel to the desired final unit; if they don’t, the setup is incorrect.
Use molar masses, Avogadro’s number, or 22.4 L/mol at STP as needed.
Use dimensional analysis (unit-cancelling chains) to organize multi-step conversions.

Stoichiometry is primarily about mole ratios from balanced equations and chaining unit conversions to link the given quantity to the wanted quantity.
The mole-to-mole conversion (using coefficients) is the core step; all other conversions (mass, particles, gas volume) are preparatory or finishing steps.
Dimensional analysis and careful unit cancellation prevent setup errors.