Chemical Quantities, The Mole, and Molar Mass – Study Notes

Ch. 7 Chemical Quantities and Reactions

The Mole Concept

The mole is a fundamental counting 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?

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

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).

• Example: H2O contains 2 hydrogen atoms and 1 oxygen atom.

• Polyatomic Ions: Parentheses indicate multiple groups, e.g., Ca(OH)2 has 2 hydroxide ions.

Sample Problem: How many H atoms are in (NH4)2SO4?

• Each NH4+ has 4 H; there are 2 NH4+ ions: $2 \times 4 = 8$ H atoms.

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 with units of g/mol.

• Calculation Steps:

  1. Write the chemical formula and count 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: Calculate the molar mass of MgSO4:

$1\ \text{Mg} \times 24.31\ \text{amu} = 24.31\ \text{amu}$ $1\ \text{S} \times 32.06\ \text{amu} = 32.06\ \text{amu}$ $4\ \text{O} \times 16.00\ \text{amu} = 64.00\ \text{amu}$ Total: $24.31 + 32.06 + 64.00 = 120.37\ \text{amu}$ Molar mass: $120.37\ \text{g/mol}$

Example 2: Calculate the molar mass of H2O:

$2\ \text{H} \times 1.01\ \text{amu} = 2.02\ \text{amu}$ $1\ \text{O} \times 16.00\ \text{amu} = 16.00\ \text{amu}$ Total: $2.02 + 16.00 = 18.02\ \text{amu}$ Molar mass: $18.02\ \text{g/mol}$

Example 3: Calculate the molar mass of CaCl2:

$1\ \text{Ca} \times 40.08\ \text{amu} = 40.08\ \text{amu}$ $2\ \text{Cl} \times 35.45\ \text{amu} = 70.90\ \text{amu}$ Total: $40.08 + 70.90 = 110.98\ \text{amu}$ Molar mass: $110.98\ \text{g/mol}$

Writing Chemical Formulas for Molecular Compounds

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

• Example: Dihydrogen sulfide

• "Di-" means 2 hydrogens; "sulfide" means 1 sulfur.

• 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.

• Example: If a metal forms a compound XF3, the metal must have a +3 charge (X3+), so it is from Group 3A.

Lecture Schedule and Course Structure

The course covers foundational topics in chemistry, including measurements, matter and energy, atomic structure, chemical bonding, chemical reactions, and the basics of organic chemistry.

Week	Chapter	Topic
1	Ch. 1	Chemistry in Our Lives
2	Ch. 2	Chemistry and Measurements
3	Ch. 3	Matter and Energy
4	Ch. 4	Atoms and Elements
5	Ch. 5	Nuclear Chemistry
6	Ch. 6	Ionic and Molecular Compounds
7	Ch. 7	Chemical Quantities and Reactions
8	Ch. 8	Gases
9	Ch. 9	Solutions
11	Ch. 11	Introduction to Organic Chemistry: Hydrocarbons

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 the flower bouquet, animal, and colored wall are not directly relevant to the chemistry content and are therefore excluded. Only images that reinforce chemical concepts (e.g., counting units, periodic table, chemical formulas, and course schedule) are included.