CHAPTER 3.2
Announcement
Top Hat Code: (Details not provided)
Test 1:
Date: 2/11
Format: Closed book, in person in CNS 128
Preparation:
Review all lecture slides
“To know” documents for the first three chapters (available in Teams)
Use practice exam 1 (available in Teams) to assess preparedness.
Lecture Presentation
Chapter 3: Chemical Reactions & Stoichiometry
Concept: Stoichiometry Made Easy
Example Problem: How many slices of bread do you need to make 7 grilled cheese sandwiches?
Topics Covered
3.1-3.2: Equations for chemical reactions; reaction patterns.
3.3: Formula & molecular weight; percentage composition.
3.4: Avogadro’s number and the moles, molar mass.
3.5: Empirical Formulas from Analysis.
3.6: Molar masses in chemical equations.
3.7: Limiting reactants, theoretical yield, percent yield.
Avogadro’s Number Illustration
Analogy: A wealthy person (richest in the world) buying water molecules at $0.01 each.
Example calculation implies his assets allow him to buy a vast number of molecules due to vast purchasing power (1 = 1.67×10−24).
19th vs 21st Century Scientist Approach
19th Century: A quest for understanding phenomena in nature.
21st Century: More focus on producing results that fit personal narratives or biases.
Understanding Avogadro’s Number and Moles
Comparison of real-world groupings (Dozen, Gross, Ream) to the concept of a mole.
Definition of Mole:
The mole connects atomic weight to measurable quantities.
1 mole is defined as 6.02 x 10^23 particles (Avogadro's number).
Molar Mass Calculation
Exchange between atomic mass units (amu) and grams:
1 amu corresponds to 1 gram per mole of a substance.
Example conversions:
1 mol O2(g): 32.0 g
1 mol H2O(l): 18.0 g
1 mol NaCl(s): 58.45 g
Notes on Mole Calculations
Types of Calculations:
Single Step:
Molecules to moles and vice versa (rare in lab, useful for exercises).
Moles to grams and vice versa (common in lab).
Multiple Steps:
Grams to molecules and vice versa (less common, good for exercises).
Example Problems in Mole Calculations
Converting between moles and grams:
Moles to Grams:
5.30 moles of water to grams: 5.30 moles x 18.0 g/mol = 95.4 g.
Grams to Moles: 95.4 g water to moles: 95.4 g x (1 mol/18.0 g) = 5.30 moles.
Empirical and Molecular Formulas
Molecular Formula: Shows actual number of atoms in a molecule.
Empirical Formula: Gives relative number of atoms; uses smallest whole-number ratios.
Steps to Calculate Empirical Formulas
Convert mass percentages to grams (assuming a 100 g sample).
Convert grams to moles.
Calculate molar ratios and derive empirical formula.
Example on Calculating Empirical Formulas
Given Compound: 73.9% Hg and 26.1% Cl.
Steps:
73.9 g Hg, 26.1 g Cl in 100 g sample.
Convert grams to moles and find mole ratios.
Calculation of Molecular Formulas
Use empirical formula to find empirical mass.
Divide molar mass by empirical mass to find integer multiplier.
Adjust subscripts to derive the molecular formula from the empirical.
Combustion Analysis Comments
Utilizes elemental composition derived from combustion to find empirical and molecular formulas.
Key Molar Mass and Relationships
The relationship between molar mass and Avogadro’s number connects grams, moles, and particles.
One mole of different entities corresponds with respective molar masses.
Practice with Chemical Equations
Understanding and executing mole conversions using balanced chemical equations.
Example: From grams to moles, and vice versa through molar masses.
Summary of Mole Calculation Concepts
Follow the pattern of mass conversions through moles using established coefficients from balanced equations.
Important steps include:
Read the balanced equation.
Convert mass to moles using molar mass.
Utilize mole/mole factors from the equation to find desired particles.
Convert back to grams if required.