Fundamental Laws of Chemistry

Conservation of Mass

The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

• Key Point: All atoms present in the reactants are accounted for in the products.

• Example: In the reaction , the total mass of hydrogen and oxygen before and after the reaction remains the same.

Law of Constant Composition

This law states that a given compound always contains the same proportion of elements by mass.

• Key Point: Water () always contains 2 hydrogen atoms for every 1 oxygen atom, regardless of the source.

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

• Example: Carbon and oxygen form CO and CO2. The ratio of oxygen masses that combine with a fixed mass of carbon is 1:2.

Atomic Structure

Subatomic Particles

Atoms are composed of three main subatomic particles:

• Protons: Positively charged particles found in the nucleus. Mass ≈ 1 amu.

• Neutrons: Neutral particles in the nucleus. Mass ≈ 1 amu.

• Electrons: Negatively charged particles orbiting the nucleus. Mass ≈ 0.0005 amu.

Nuclear Atom

The nuclear model of the atom describes a dense, positively charged nucleus containing protons and neutrons, surrounded by electrons in a cloud.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

• Percent Abundance: The relative amount of each isotope in a natural sample.

• Calculation of Atomic Weight: The average atomic mass is calculated using the percent abundance and mass of each isotope:

• Example: If is 75% and is 25%, then

The Periodic Table

Families (Groups) and Periods

• Families (Groups): Vertical columns; elements in the same group have similar chemical properties.

• Periods: Horizontal rows; properties change progressively across a period.

Example: Group 1: Alkali metals; Group 17: Halogens.

Name That Element

• Each element is identified by its atomic number (number of protons).

The Mole Concept

Avogadro’s Number

One mole contains entities (atoms, molecules, ions, etc.).

Molar Mass

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

• Example: Molar mass of is g/mol.

Types of Compounds

• Ionic Compounds: Composed of metals and nonmetals; transfer of electrons.

• Covalent Compounds: Composed of nonmetals; sharing of electrons.

Percent Composition of Compounds

The percent by mass of each element in a compound.

• Example: In , %H =

Empirical Formula

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

• Calculation from % by Mass or Masses: Convert mass to moles, divide by smallest number of moles, round to nearest whole number.

• Combustion Analysis: Used to determine empirical formula from combustion products (covered in a later quiz).

Balancing Chemical Equations

Ensures the same number of each atom on both sides of the equation, in accordance with the Law of Conservation of Mass.

• Steps: Write correct formulas, count atoms, adjust coefficients, check work.

• Example:

Stoichiometry: Mass and Mole Calculations

Mass to Mole to Mole Calculations

Stoichiometry involves using balanced equations to relate masses and moles of reactants and products.

• Simple Stoichiometry: Use molar ratios from the balanced equation.

• Example: How many moles of are produced from 2 moles of ?

Solutions: Molarity and Dilution

Molarity

Molarity (M) is the concentration of a solution, defined as moles of solute per liter of solution.

Dilution

To dilute a solution, add solvent to decrease concentration. The number of moles of solute remains constant.

• Example: To make 250 mL of 0.1 M solution from 1.0 M stock: ; L = 25 mL.

Significant Figures

Rules for determining the number of meaningful digits in a measurement or calculation.

• Nonzero digits are always significant.

• Zeros between nonzero digits are significant.

• Leading zeros are not significant.

• Trailing zeros are significant only if there is a decimal point.

Dimensional Analysis

A method for converting between units using conversion factors.

• Set up conversion factors so units cancel appropriately.

• Example: Convert 25.0 g to moles for a substance with molar mass 50.0 g/mol:

Additional info: Combustion analysis is mentioned as a later topic, so only a brief note is included here. Nomenclature is excluded as per the original file.