Principles of Chemistry Unit 2

Formula mass

The sum of the average atomic masses of all atoms in a chemical formula.

Difference between molecular mass and formula mass

Molecular mass is for covalent compounds; formula mass is for ionic compounds.

Mole

1 mole = 6.022 × 10^23 entities (Avogadro’s number).

Molar mass

Mass of 1 mole of a substance in g/mol, equal to its formula mass in amu.

Convert grams to moles

Divide grams by molar mass.

Convert moles to grams

Multiply moles by molar mass.

Percent composition

(mass of element ÷ mass of compound) × 100.

Empirical formula

Simplest whole-number ratio of atoms in a compound.

Molecular formula

Actual number of atoms in a molecule.

Finding molecular formula from percent composition

Find empirical formula, then multiply by factor = (molecular mass ÷ empirical mass).

Molarity (M)

Moles of solute ÷ liters of solution.

Dilution formula

M1V1 = M2V2.

Other concentration units

Mass percent, ppm, ppb.

Example: NaOH solution

To make 250 mL of 12.1 M NaOH, you need 121 g NaOH.

Chemical equation

A symbolic representation of a chemical reaction with reactants and products.

Why balance equations

To follow the Law of Conservation of Mass.

Steps to balance

1. Write unbalanced equation. 2. Balance atoms with coefficients. 3. Check. 4. Add states.

Example: Balance CH4 + O2

CH4 + 2O2 → CO2 + 2H2O.

Limiting reactant

Reactant used up first, limiting the product formed.

Theoretical yield

Max product predicted from stoichiometry.

Percent yield

(actual ÷ theoretical) × 100.

Spectator ions

Ions unchanged on both sides of an ionic equation.

Net ionic equation

Shows only the species that actually change.

Three main reaction types

Precipitation, Acid-Base, Redox.

Precipitation reaction

Two aqueous solutions form an insoluble solid.

Solubility rules

Always soluble: Group 1, NH4+, NO3-, CH3COO-. Halides soluble except Ag, Pb, Hg. Sulfates soluble except Ba, Sr, Ca, Pb, Ag. Insoluble: CO3^2-, PO4^3-, OH-, S^2-, O^2- (unless with Group 1 or NH4+).

Acid-base reaction

Acid + base → water + salt.

Strong acids

HCl, HBr, HI, HNO3, H2SO4.

Strong bases

NaOH, KOH, Ca(OH)2.

Gas-evolution reaction

Reaction that produces a gas (CO2, H2S, NH3).

Oxidation

Loss of electrons, increase in oxidation number.

Reduction

Gain of electrons, decrease in oxidation number.

Oxidation number rules

Free elements = 0. Monatomic ions = charge. O = –2 (except peroxides, F2O). H = +1 (–1 in hydrides). F = –1 always. Group 1 = +1, Group 2 = +2. Sum = charge of species.

Reducing agent

Species oxidized (loses electrons).

Oxidizing agent

Species reduced (gains electrons).

Combustion reaction

Reaction with O2 producing CO2 + H2O (redox).

Example: AgNO3 + NaCl

AgCl(s) + NaNO3(aq). Precipitation reaction.

Example: HCl + NaOH

H2O(l) + NaCl(aq). Acid-Base neutralization.