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Understanding Mole Concept and Formula Mass
Mar 6, 2025
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Chapter 3: Composition of Substances and Solutions
Introduction
Chapter 3 and 4 are heavily math-focused in general chemistry.
Dimensional analysis from Chapter 1 is crucial.
Main question: how do we calculate and perform math with molecular compositions?
Subchapters
3.1 Formula Mass and the Mole Concept
3.2 Empirical and Molecular Formulas
3.3 Molarity in Solutions
**3.4 Other Units
3.1 Formula Mass and Mole Concept
Formula Mass:
Sum of average atomic masses of all atoms in a compound.
Atomic mass = mass of one atom
Formula mass (or molecular mass) = mass of a molecule or covalent compound.
Example:
Chloroform (CHCl₃)
Formula: 1 Carbon, 1 Hydrogen, 3 Chlorines
Calculate: Add atomic masses (C: 1(12), H: 1(1), Cl: 3(35).)
Result: 119.37 amu (atomic mass units)
Calculating Formula Mass
Aspirin (C₉H₈O₄)
Formula: 9 Carbons, 8 Hydrogens, 4 Oxygens
Calculate: Sum of each element’s atomic masses
Result: 180.15 amu
Ionic Compounds
Calculate similarly to molecular mass but termed as formula mass
Example: Sodium Chloride (NaCl)
The Mole Concept
Mole:
Amount of substance; Avogadro's number = 6.022 × 10²³ units/mole.
Converting Mass and Moles:
Molar mass (g/mol) is numerically equal to formula mass (amu).
Formula mass helps in calculating molar mass.
Examples:
1 mole of Zinc = 65.4 g
1 mole of Calcium = 40.08 g
Units:
Grams per mole (g/mol)
Mathematical Relationships
Conversions:
Grams to moles using molar mass.
Moles to atoms using Avogadro's number.
Dimensional Analysis:
Critical for tracking units and ensuring correct calculations.
Practice Problems
Convert grams to moles and vice versa.
Calculate formula mass for molecules and ionic compounds.
Example problems provided for hands-on practice (e.g., Potassium, Argon).
Key Takeaways
Conversion Factors:
Use them to relate mass, moles, and number of atoms.
Molar Mass and Formula Mass:
Numerically equal but used in different contexts (atomic vs macroscopic scale).
Consistent Practice:
Essential for mastering concepts in chemistry.
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