Overview
This lecture covers advanced topics in reaction stoichiometry, specifically focusing on stoichiometry involving solutions (using molarity) and gases (using the ideal gas law), with detailed example problems.
Solution Stoichiometry
- Molarity measures the concentration of a solute in a solution as moles solute per liter of solution.
- To find moles of solute: moles = molarity × volume (L); always convert mL to L.
- Limiting reactant determines the maximum amount of product formed in a reaction.
- In reactions, balance the chemical equation before performing calculations.
- For example: 2.5 g magnesium reacts with 50 mL of 1.20 M HCl to yield MgCl₂; calculate moles for HCl and Mg, check limiting reactant, and convert moles of product to grams.
- Always choose the smallest calculated product mass based on the limiting reactant.
- Double displacement reactions form products by swapping ions between reactants (e.g., CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl).
- For each reactant, calculate moles present, determine limiting reactant, use stoichiometry to find product moles, and convert to grams with molecular weight.
Gas Stoichiometry
- For gases, use the ideal gas law: PV = nRT, where n = moles, R = 0.08206 L·atm/mol·K, T in Kelvin.
- To solve for gas volume: V = (nRT)/P.
- Example: Given moles of H₂ produced, use the ideal gas law to calculate volume at specified temperature and pressure.
- When given mass of a solid reactant (e.g., CaC₂), convert mass to moles, use balanced reaction for moles of gas produced, then use the ideal gas law for volume at given conditions.
Key Terms & Definitions
- Molarity (M) — moles of solute per liter of solution.
- Limiting Reactant — the reactant that limits the amount of product in a chemical reaction.
- Ideal Gas Law — equation (PV = nRT) relating pressure, volume, temperature, and moles of a gas.
- Stoichiometry — calculation of reactants and products in chemical reactions.
Action Items / Next Steps
- Practice more stoichiometry problems involving solutions and gases.
- Review the calculation steps for identifying the limiting reactant and applying the ideal gas law.
- Watch for additional example solutions as suggested in the video.