Calculations with Chemical Formulas and Equations

Molecular Mass

The molecular mass is the sum of the atomic masses of all the atoms in a molecule of a substance. It is a fundamental property used to characterize molecules and is essential for quantitative chemical calculations.

• Definition: The total mass of all atoms in a single molecule, expressed in atomic mass units (amu).

• Example: For water (H2O): amu

Formula Mass

The formula mass is the sum of the atomic masses of all atoms in a formula unit of a compound, whether the compound is molecular or ionic.

• Definition: The total mass of all atoms in a formula unit (used for ionic compounds).

• Example: For sodium chloride (NaCl): amu

Calculating Formula Weights

To calculate the formula weight, sum the atomic masses of each atom in the chemical formula, multiplying by the number of times each atom appears.

• Calcium hydroxide, Ca(OH)2: Ca: O: H: Total: amu (rounded to three significant figures: 74.1 amu)

• Methylamine, CH3NH2: C: H: N: Total: amu (rounded to three significant figures: 31.1 amu)

Mole and Avogadro's Number

The mole (mol) is a quantity that represents 6.022 × 1023 particles (atoms, molecules, or formula units). This number is known as Avogadro's Number (NA).

• Definition: 1 mole contains as many entities as there are atoms in exactly 12 g of carbon-12.

• Avogadro's Number:

Molar Mass

The molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol).

• Example: Carbon-12 has a molar mass of 12 g/mol.

• Formula:

Conversions Using Molar Mass

To convert between grams and moles, use the molar mass as a conversion factor.

• Example: A sample of nitric acid (HNO3) contains 0.253 mol. The molar mass of HNO3 is g/mol.

Calculating Moles from Mass

To find the number of moles in a given mass of a compound, divide the mass by the molar mass.

• Example: For 23.6 g of CaCO3 (molar mass = 100.09 g/mol):

Daily Requirements and Moles

Chemical quantities in nutrition and medicine are often expressed in moles for precise measurement.

• Leucine (C6H14O2N): If the daily requirement is 2.25 g, and the molar mass is 132.18 g/mol:

• Chromium: For 1.0 × 10-6 g, and atomic mass 51.996 g/mol: atoms

Percentage Composition

The percentage composition of a compound is the mass percentage of each element present in the compound.

• Definition:

• Example (PbCrO4):

• Example (Sucrose, C12H22O11): Molar mass = 342.30 g/mol C: H: O: To find grams of C in 3.1 g sucrose:

Experimental Determination of Composition

Percentage composition can be determined by burning a compound and measuring the masses of the resulting oxides (e.g., CO2 and H2O).

• Strategy:

  1. Use mass of CO2 to find mass of carbon.

  2. Use mass of H2O to find mass of hydrogen.

  3. Calculate percent composition from these masses.

*Additional info: These notes cover the first half of Chapter 3, focusing on chemical formulas, molar mass, mole concept, and percentage composition. Later sections (not shown in images) likely cover empirical/molecular formulas and stoichiometry, as indicated in the text transcript.*