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Stoichiometry

by | Feb 7, 2024

Notes

  • Reaction Stoichiometry: mass relationships between reactants and products in a chemical reaction

  • Mole Ratio: conversion factor that relates amount in moles of two substances in chemical reaction

  • Coefficients convey ratio of substances needed for the reaction to occur (in terms of moles)
    • One mole water = 18 grams; 1 molecule of water = 18 amu
    • Oxygen Ratio: 1 mole = 16 grams = 6.02 x 10^23 atoms
      • Round up to 100th place
  • Ex: 4:5:6:4 can mean 4 molecules react with 5 molecules to produce 6 molecules and 4 molecules

Molar Mass

Molar Mass of a Compound

  • Determine how many of each atom

  • Multiply each element’s number of atoms by atomic mass

  • Add up total mass to find molar mass (don’t forget to write grams/mol)

Molar Mass of a Compound Containing Polyatomic Ions

    • Number under parenthesis belongs to ALL atoms so multiply to find how many

Determining the Formula of a Compound

    • Molecular Formula: the actual number of atoms of each element in a compound

      • Ex: C6H12O6

    • Empirical Formula: the lowest whole number ratio of elements

      • Ex: : CH2O

Determining Empirical and Molecular Formula for a Compound

  • Use process when asked to find subscripts of an element in compound
  1. Multiply given percent with element’s molar mass to find gram of element → convert grams into moles (divide each element by atomic mass) → divide by lowest number of moles = empirical formula
  2. If given only percentages, assume 100g sample and convert to grams → follow above steps
  3. If you do not have a whole number, multiply by the smallest whole # to get a whole # ratio → whole #s = subscripts for each element
  4. To find the molecular formula, divide the molecular molar mass by the empirical molar mass. Then multiply each of the subscripts by that answer

Empirical Formula on the AP Exam

    1. Straightforward mass or % composition data
    2. Combustion analysis: burning a sample in O2 and analyzing products to determine relative amounts of C and H
    3. Hydrate analysis: heating hydrated ionic solid and analyzing mass change to determine mole ratio of water to solid
      • Hydrate is heated → water is driven off → mass of sample decreases; hydrate is no longer a hydrate when mass is not decreasing
      • To determine formula of hydrated compound → determine moles of water → determine moles of hydrate → find ratio of moles of water / moles of compound → answer equals the number of H2O

The Meaning of a Chemical Equation

  • The chemical equation for a reaction gives 2 types of info: the nature of the reactants and products and the relative numbers of each.

Balancing Chemical Equations

  • Atoms are conserved in a chemical reaction.
  • Only the coefficients can be changed; the subscripts in a formula cannot be changed, nor can atoms be added or subtracted from a formula.
  1. Process in balancing elements: Metals → Nonmetals → Hydrogen → Oxygen

For Combustion Reactions (Fuel + O2 → CO2 + H2O)

  1. Balance carbon → hydrogen
  2. Count the number of oxygen atoms on the right
  3. Take half that number and make it the coefficient of the oxygen on the left
  4. If you now have a fraction, multiply all coefficients by 2

Stoichiometric Calculations: Amounts of Reactants and Products

    • Moles to particles → use avogadro’s number
    • Moles to mass (grams) → use molar mass from periodic table
    • Moles to Volume (liters) → use 22.4 liters/mol but only for gasses at standard temp and pressure

Calculating Masses of Reactants and Products in Reactions

  1. Balance the equation for the reaction
  2. Convert the known masses of reactant or product to moles
  3. Use balanced equation to set up mole ratios
  4. Use the mole ratios to calculate the number of moles of desired reactant or product.
  5. If required, convert moles back to grams

The Limiting Reactant

  • Limiting Reactant: the one that is consumed first and thus limits the amount of product

  • You know you have a limiting reactant problem anytime you are given amounts of both reactants
  • The reactant that is limiting will make the least amount of product

  • Theoretical yield: The amount of a product formed when the limiting reactant is completely consumed

    • What you should have gotten if everything was perfect

  • Actual yield: what you actually get from a reaction

  • AP exam will never give you the theoretical yield → will always have to calculate it
    • Given mass of reactant
      1. Write balanced equation
      2. Process: mass of reactant given x (grams of product/grams of reactant from balanced equation

Determination of Limiting Reactant Using Reactant Quantities

  1. Balance the equation
  2. Do two stoichiometry problems
  3. Figure out how much product each reactant makes
  4. The one that makes the least is the limiting reagent (the other is the excess reagent)
  5. The lesser amount of product is the true amount made