Chemical Quantities and the Mole Concept

Definition of the Mole

The mole is a fundamental unit in chemistry used to express amounts of a chemical substance. One mole contains exactly Avogadro's number of particles (atoms, molecules, or formula units).

• Avogadro's Number: particles per mole

• Application: Used to count atoms, molecules, or ions in a given sample.

• Example: 1 mole of sodium (Na) contains Na atoms.

Atomic Mass and Molar Mass

Atomic mass is the mass of a single atom, usually expressed in atomic mass units (amu). Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol).

• Atomic Mass of Na: 22.99 amu

• Molar Mass of Na: 22.99 g/mol

• Relationship: The numerical value of atomic mass in amu is equal to the molar mass in g/mol.

• Example Calculation: To find the mass of 1 mole of Na atoms:

  • Number of atoms in 1 mole:

  • Mass of 1 Na atom: g (calculated as )

  • Total mass: g for atoms

Converting Between Grams, Moles, and Number of Particles

Conversions between mass, moles, and number of particles are fundamental in chemical calculations.

• Key Relationships:

• Example 1: Convert 5.00 mol Na to grams:

• Example 2: Convert 500 g K to moles and atoms:

  • Molar mass of K = 39.10 g/mol

  • Number of atoms:

Formula Mass and Molar Mass of Compounds

Definition and Calculation

The formula mass (for ionic compounds) or molecular mass (for molecular compounds) is the sum of the atomic masses of all atoms in a chemical formula. The molar mass of a compound is the mass of one mole of that compound, expressed in g/mol.

• Formula:

• Example 1: For NaCl:

  • Na: 1 atom × 22.99 amu = 22.99 amu

  • Cl: 1 atom × 35.45 amu = 35.45 amu

  • Total: amu

  • Molar mass: 58.44 g/mol

• Example 2: For H2O:

  • H: 2 × 1.008 amu = 2.016 amu

  • O: 1 × 16.00 amu = 16.00 amu

  • Total: amu

  • Molar mass: 18.016 g/mol

Types of Compounds and Counting Units

• Element: 1 mole = atoms

• Molecular Compound: 1 mole = molecules

• Ionic Compound: 1 mole = formula units

Percent Composition by Mass

Definition and Calculation

Percent composition by mass is the percentage by mass of each element in a compound. It is calculated as follows:

• Formula:

• Example: For C2H6O (ethanol):

  • Molar mass: g/mol

  • Mass % C:

  • Mass % H:

  • Mass % O:

Application: Calculating Mass of an Element in a Sample

• Example: How many grams of C are in 200 g of C2H6O?

  • Mass % C = 52.2%

  • Mass of C =

Summary Table: Types of Compounds and Counting Units

Key Formulas

Additional info: Some calculations and values were clarified and expanded for completeness and clarity. The notes include standard definitions and examples for General Chemistry students.