Topic 6 Stoichiometry

Vocabulary:

atomic mass unit (amu) - a unit of mass used to express atomic and molecular weights

Average atomic mass - the weighted average of the atomic masses of an element's isotopes

Mole - a unit defined as the amount of substance that contains the same number of entities (a thing with distinct and independent existence) as there are in 12 grams of carbon-12

Avogadro's number - the number of particles in one mole of a substance

Molar mass - the mass of one mole of a substance in grams per mole (g/mol)

Percentage composition: The percentage by mass of each element in a compound.

Empirical formula: Consists of element symbols with subscripts indicating the smallest whole-number ratio of different atoms.

Molecular formula: Indicates the actual number of atoms of each element in a molecule of a substance.

Stoichiometry: The study of the mass relationships between reactants and products in chemical reactions.

Mole ratio: A conversion factor for mole relationships derived from balanced equations.

Limiting reactant: The reactant that is used up first and limits product formation in a chemical reaction.

Excess reactant: The reactant that remains after the reaction has completed.

Theoretical yield: The maximum amount of product that can be formed from given reactants under ideal conditions.

Actual yield: The measured amount of a product obtained from a chemical reaction.

Percentage yield: The ratio of actual yield to theoretical yield, expressed as a percentage.

Concept A

Definition of molar mass

• Number of grams per one mole of a substance

• Whole number ratio that is a multiple of a chemical formula

• Simplest whole number ratio of a chemical formula

6.022 x 10^23 particles per one mole of a substance

Relative atomic mass:

• Atomic masses are expressed in relative atomic mass units (amu).

• 1 amu = 1/12 the mass of a carbon 12-atom

Carbon-12 has 6 protons and 6 neutrons, each with a mass of about 1.0 amu.

Average Atomic Mass:

A weighted average of an element's isotopes, found on the periodic table.

Mole:

• An SI unit measuring the amount of substance; 1 mole contains 6.022 x 10^23 particles (Avogadro's number).

• Equivalent to the number of particles in 12g of carbon-12.

Molar Mass:

• Mass of one mole of a pure substance; units are g/mol.

• Example: Molar mass of Lithium = 6.94 g/mol, equivalent to 6.94 amu.

  Concept B

• The percentage by mass of each element in a compound is known as the percentage composition.

  Concept C

• Empirical Formula Definition

  • Definition: Empirical formula consists of element symbols with subscripts indicating the smallest whole-number ratio of different atoms.

  • Use of Data: Percentage composition data is utilized to derive the empirical formula.

Concept D : Introduction to Stoichiometry

• Vocabulary: Stoichiometry, mole ratio.

• Objectives: Analyze balanced equations for mole ratios and calculate quantities involved in reactions.

Page 12 - Basic Stoichiometric Calculations

• Question Example: From the equation Mg + 2HCl -> MgCl2 + H2, how many moles of MgCl2 result from 4 mol HCl?

Page 14 - Understanding Stoichiometry

• Definition: Stoichiometry studies the mass relationships between reactants and products.

• 

  • Mole Ratio: A conversion factor for mole relationships.

  • Sample Problem: Balance Al2O3 -> Al + O2 and write mole ratios.

Page 15 - Problem Types in Stoichiometry

• Four Problem Types:

  1. Amounts in moles.

  2. Moles to grams.

  3. Grams to moles.

  4. Grams to grams via moles.

concept e

Concept E: Identifying Reactants

• Objectives: Identify limiting and excess reactants, determine excess remaining.

• Key Vocabulary: Limiting reactant, excess reactant.

Page 17 - Reactant Identification Example

• Question Example: Given reaction C + O2 -> CO2, determine limiting reactant with provided amounts.

Page 18 - Limiting vs. Excess Reactants

• Limiting Reactant: Used up first and limits product formation.

• Excess Reactant: Remains after the reaction completes.

• 

  • Example Problem: Silicon dioxide and hydrogen fluoride reaction.

Page 19 - Further Examples

• Problem Example: Reaction of Fe and H2O producing Fe3O4; determine limiting reactant, mass produced, and excess remaining.

• Concept F: Percentage Yield

  • Objectives: Distinguish between theoretical and actual yield, calculate percentage yield.

  • Key Vocabulary: Theoretical yield, actual yield, percentage yield.

  Page 20 - Defining Yield

  • Theoretical Yield: Maximum product possible from reactant amounts.

  • Actual Yield: Measured amount from a reaction.

  • 

  • Percentage Yield: Ratio of actual to theoretical yield.

    

  Example of Yield Calculation

  • Sample Problem: Calculate percentage yield for chlorobenzene from given reactants.

  Page 24 - Practical Yield Calculations

  • Additional Example Problems:

    1. Determine percentage yield of methanol from carbon monoxide and hydrogen gas.

    2. Calculate mass of copper produced from aluminum reacting with copper(II) sulfate.