Chemistry Unit 8

Molar Mass

  • The mass of one mole of a substance (atoms, molecules, or formula units) expressed in grams/mole.

  • To calculate, sum the atomic masses of all atoms in the chemical formula.

    • Example: Molar mass of H_2O = 2(1.01 g/mol) + 1(16.00 g/mol) = 18.02 g/mol

Mole Conversion

  1. Mole to Mass:

    • Use molar mass as a conversion factor.

    • Mass (grams) = Moles \times Molar Mass (g/mol)

  2. Mass to Mole:

    • Use the inverse of molar mass.

    • Moles = Mass (grams) / Molar Mass (g/mol)

  3. Mole to Particles:

    • Use Avogadro's number (6.022 \times 10^{23}).

    • Particles = Moles \times Avogadro's Number

  4. Particles to Mole:

    • Use the inverse of Avogadro's number.

    • Moles = Particles / Avogadro's Number

Mole Concept

  • A mole is a unit of amount (n) equal to Avogadro's number of particles.

    • Avogadro's Number: 6.022 \times 10^{23}

Percent Composition

  • The percent by mass of each element in a compound.

  • % Composition = (Mass of Element / Mass of Compound) \times 100%

Empirical and Molecular Formulas

  1. Empirical Formula:

    • The simplest whole-number ratio of atoms in a compound.

  2. Molecular Formula:

    • The actual number of atoms of each element in a molecule.

    • (Empirical Formula) \times n = Molecular Formula, where n is an integer.

Steps to Determine Empirical Formula:

  1. Convert % composition to grams (assume 100g sample).

  2. Convert grams to moles.

  3. Divide all mole values by the smallest mole value to get the simplest ratio.

  4. If necessary, multiply to get whole numbers.

    • Example: C{2}H{4}O{2}, Empirical Formula: CH{2}O

Steps to Determine Molecular Formula:

  1. Calculate the molar mass of the empirical formula.

  2. Divide the molar mass of the molecular formula by the molar mass of the empirical formula to find n.

  3. Multiply the subscripts in the empirical formula by n.

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