Unit+5

Chapter Overview

  • Chapter 5: The Mole and Chemical Formulas

    • 5.1 Counting Atoms by Weighing

    • 5.2 Counting Molecules by Weighing

    • 5.3 Mass Percent Composition

    • 5.4 Using Mass Percent Composition to Determine Empirical Formula

    • 5.5 Using Empirical Formula and Molar Mass to Determine Molecular Formula

The Mole

  • Atoms are minute, and even a small quantity contains countless atoms.

  • Chemists use a unit called the mole (mol) for convenience.

Avogadro’s Number

  • The mole is equivalent to 6.022 × 10²³ items, known as Avogadro’s number (NA).

  • Defined as the number of carbon atoms in exactly 12 grams of carbon-12.

  • A mole of a substance contains 6.022 × 10²³ of its particles.

Counting Atoms by Weighing

  • The number of atoms in a sample can be determined by weighing it.

  • The mass of one mole of a substance is called its molar mass.

Sample Calculations

  • Example: For helium (He), calculate the number of atoms using molar mass and Avogadro’s number.

Interconverting Mass and Moles

  • Formula for converting between mass and moles:

    • [ \text{moles of element} = \frac{\text{mass of element (g){\text{molar mass of element (g/mol) ]

    • Rearranging this can help in finding mass or moles from given parameters.

Molecular Mass and Counting Molecules

  • Molecular Mass: The total of atomic masses for all atoms in a molecule.

  • Example: To find the formula mass of NaCl by summing atomic masses.

Molar Mass Calculation

  • To determine molar mass, sum the molar masses of each element in the compound.

  • Example: Water (H2O) and sodium chloride (NaCl).

Sample Problems on Counting Molecules

  • Problems include finding moles from grams of carbon, helium, sodium, and calculating number of atoms.

Mass Percent Composition

  • The percent of total mass contributed by each element in a compound is called mass percent composition.

  • Formula:

    • [ \text{percent by mass} = \frac{n \times \text{molar mass of element}}{\text{molar mass of compound}} \times 100% ]

Sample Calculation

  • Example: For H2O2, calculate mass percent composition of hydrogen and oxygen.

Determining Empirical Formula

  • Empirical formula represents the simplest whole number ratio of atoms in a compound.

  • Convert mass percent composition to grams (e.g., assume 100 g total).

  • Find moles of each element, then simplify the ratio to get the empirical formula (e.g., C7.682H7.682 → CH).

Examples

  • Given mass compositions, determine empirical formulas for compounds like carbon and hydrogen.

Using Empirical Formula and Molar Mass to Find Molecular Formula

  • Empirical formulas indicate ratios, while molecular formulas give exact counts of atoms.

  • To find molecular formula:

    • [ n = \frac{\text{molar mass}}{\text{molar mass of empirical formula}} ]

  • Example: Empirical formula CH with molar mass 78 g leads to C6H6.

Final Sample Problems

  • Determine empirical formulas from given mass percent compositions and find molecular formulas from given molar masses.