Chemical Quantities, The Mole, and Molar Mass: Study Notes for General Chemistry

Chemical Quantities and Reactions

The Mole Concept

The mole is a fundamental unit in chemistry used to express amounts of a chemical substance. It allows chemists to count atoms, molecules, or ions by weighing them.

• Definition: One mole (abbreviated as mol) contains exactly $6.02 \times 10^{23}$ particles (Avogadro’s number).

• Analogy: Just as a dozen means 12 items, a mole means $6.02 \times 10^{23}$ items.

• Application: 1 mole of H2O molecules = $6.02 \times 10^{23}$ H2O molecules.

• Conversion Factors: The mole can be used to convert between the number of particles and the amount in moles:

$1\ \text{mol} = 6.02 \times 10^{23}\ \text{atoms, molecules, or ions}$

• Example Calculation: How many molecules are in 3.0 moles of NaF?

$3.0\ \text{mol} \times \frac{6.02 \times 10^{23}\ \text{molecules}}{1\ \text{mol}} = 1.8 \times 10^{24}\ \text{molecules}$

• Reverse Calculation: How many moles are in $2.11 \times 10^{24}$ atoms of H2?

$2.11 \times 10^{24}\ \text{atoms} \times \frac{1\ \text{mol}}{6.02 \times 10^{23}\ \text{atoms}} = 3.50\ \text{mol}$

• Key Point: Always select the conversion factor with the desired unit in the numerator.

Chemical Formulas and Formula Mass

Chemical formulas represent the types and numbers of atoms in a compound. The formula mass is the sum of the atomic masses of all atoms in the formula, measured in atomic mass units (amu).

• Examples:

  • H2O: 2 hydrogen atoms, 1 oxygen atom

  • NaCl: 1 sodium atom, 1 chlorine atom

  • Ca(OH)2: 1 calcium ion, 2 hydroxide ions

• Counting Atoms: For (NH4)2SO4:

  • 2 ammonium ions (NH4+) = 2 N, 8 H

  • 1 sulfate ion (SO42-) = 1 S, 4 O

  • Total: 2 N, 8 H, 1 S, 4 O

Molar Mass

The molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is numerically equal to the formula mass in amu, but the units are grams per mole.

• Calculation Steps:

  1. Write the correct formula and determine the number of each atom.

  2. Multiply the number of atoms by their atomic weights (from the periodic table).

  3. Add the results to get the formula mass (amu) or molar mass (g/mol).

• Example 1: MgSO4

  • 1 Mg atom × 24.31 amu = 24.31 amu

  • 1 S atom × 32.06 amu = 32.06 amu

  • 4 O atoms × 16.00 amu = 64.00 amu

  • Formula mass: 120.37 amu

  • Molar mass: 120.37 g/mol

• Example 2: H2O

  • 2 H atoms × 1.01 amu = 2.02 amu

  • 1 O atom × 16.00 amu = 16.00 amu

  • Formula mass: 18.02 amu

  • Molar mass: 18.02 g/mol

• Example 3: CaCl2

  • 1 Ca atom × 40.08 amu = 40.08 amu

  • 2 Cl atoms × 35.45 amu = 70.90 amu

  • Formula mass: 110.98 amu

  • Molar mass: 110.98 g/mol

Writing Chemical Formulas for Molecular Compounds

To write the chemical formula for a molecular compound, use the prefixes in the compound’s name to determine the number of each type of atom.

• Example: Dihydrogen sulfide

  • "Di-" means 2 hydrogen atoms

  • "Sulfide" means 1 sulfur atom

  • Formula: H2S

Periodic Table Groups and Ionic Compounds

The group number of a metal in the periodic table often determines the charge of its ion in ionic compounds.

• Group 1A: Forms +1 ions (e.g., Na+)

• Group 2A: Forms +2 ions (e.g., Mg2+)

• Group 3A: Forms +3 ions (e.g., Al3+)

• Example: If a metal forms a compound XF3, the metal must be X3+ (Group 3A)

Lecture Schedule and Course Structure

The course covers foundational topics in general chemistry, including measurements, matter, atomic structure, chemical bonding, chemical reactions, solutions, acids and bases, and an introduction to organic chemistry.

Week	Date	Chapter	Quiz Dates
1	1/13, 1/15	Ch. 1. Chemistry in Our Lives
2	1/20, 1/22	Ch. 2. Chemistry and Measurements	Quiz 1 opens due 1/29
3	1/27, 1/29	Ch. 3. Matter and Energy	Quiz 2 opens due 2/5
4	2/3, 2/5	Ch. 4. Atoms and Elements	Quiz 3 opens due 2/17
5	2/10, 2/12	Exam 1
6	2/17, 2/19	Ch. 5. Nuclear Chemistry
7	2/24, 2/26	Ch. 6. Ionic and Molecular Compounds	Quiz 4 opens due 3/3
8	3/3, 3/5	Ch. 7. Chemical Quantities and Reactions	Quiz 5 opens due 3/12
9	3/10, 3/12	Ch. 8. Gases
10	3/17, 3/19	Ch. 9. Solutions
11	3/24, 3/26	Spring Break (NO CLASS)
12	3/31, 4/2	Ch. 11. Intro to Organic Chemistry: Hydrocarbons	Quiz 6 opens due 4/9

Summary Table: Molar Mass Calculations

Compound	Molar Mass (g/mol)
NH4Cl	53.50
C2H5OH	46.08
Al2(CO3)3	233.99

Additional info: The images of flowers, colored dots, and the animal are not directly relevant to the chemical concepts discussed and are therefore excluded.