Law of Constant Composition and Chemical Formulas

Law of Constant Composition

The Law of Constant Composition states that a given chemical compound always contains its component elements in fixed ratio by mass, regardless of its source or method of preparation.

• Chemical formulas represent the types and numbers of atoms in a compound.

• Elements with more than one atom per molecule use subscripts (e.g., H2O, CO2).

• Polyatomic ions are groups of atoms that act as a single charged unit in compounds (e.g., SO42−).

Atomic Elements and Polyatomic Ions

• Common atomic elements: HOFBrINCl (Hydrogen, Oxygen, Fluorine, Bromine, Iodine, Nitrogen, Chlorine) are diatomic in their elemental form.

• Polyatomic ions:

  • SO42− (sulfate)

  • PO43− (phosphate)

  • NO3− (nitrate)

Naming Compounds

Chemical compounds are named according to specific rules based on their composition:

• Ionic compounds: Consist of cations (positive ions) and anions (negative ions). The cation is named first, followed by the anion. For example, NaCl is sodium chloride.

• Molecular compounds: Use prefixes to indicate the number of each atom (e.g., CO2 is carbon dioxide).

• Acids: Naming depends on the type of anion. Binary acids use the prefix 'hydro-' and the suffix '-ic' (e.g., HCl is hydrochloric acid). Polyatomic acids use the suffix '-ic' or '-ous' depending on the anion (e.g., H2SO4 is sulfuric acid).

Example: Name the following compounds: MgSO4 (magnesium sulfate), KCl (potassium chloride), PO4 (phosphate), H3PO4 (phosphoric acid).

Additional info: When combining elements, use the rules for ionic or molecular compounds. For example, Na and Cl combine to form NaCl, Al and O combine to form Al2O3.

Chemical Composition and Calculations

Calculating with the Mole

The mole is a fundamental unit in chemistry used to count particles (atoms, molecules, ions). One mole contains Avogadro's number of entities:

• Avogadro's number:

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

• Counting by weighing concept (Mole): Relates the mass of a sample to the number of moles present.

Conversions and Calculations

• Convert between moles, grams, and number of particles using molar mass and Avogadro's number.

• Empirical formula: The simplest whole-number ratio of atoms in a compound.

• Molecular formula: The actual number of atoms of each element in a molecule.

Example: How many moles is 6.25 × 102 g of Na2SiO3? How many grams of Si can be extracted? How many atoms of O in the compound?

Empirical formula calculation: If a compound is 63.15% C, 3.56% H, and 31.55% O, and the molar mass is 456 g/mol, calculate the molecular formula.

Chemical Reactions

Types of Chemical Reactions

Chemical reactions are classified based on how reactants change into products:

• Combination (Synthesis): Two or more substances combine to form one product.

• Decomposition: A single compound breaks down into two or more simpler substances.

• Single Displacement: One element replaces another in a compound.

• Double Displacement: Exchange of ions between two compounds.

• Combustion: A substance reacts with oxygen, releasing energy as light or heat.

Writing and Balancing Chemical Equations

• Write balanced equations to obey the Law of Conservation of Mass.

• Use solubility tables to predict products and identify precipitates in double displacement reactions.

• Complete equations based on reaction type.

Ionic Equations

• Conventional (molecular) equation: Shows all reactants and products as compounds.

• Complete ionic equation: Shows all strong electrolytes as ions.

• Net ionic equation: Shows only the species that actually change during the reaction.

Example: Write the balanced conventional and net ionic equation for iron(III) chloride reacting with aluminum wire. Write the equations for calcium nitrate reacting with sodium hydroxide.

HTML Table: Common Polyatomic Ions

The following table summarizes some common polyatomic ions and their formulas:

Summary

• Understanding chemical formulas and naming conventions is essential for describing compounds.

• Calculations involving moles, mass, and empirical/molecular formulas are foundational skills in chemistry.

• Classifying and balancing chemical reactions helps predict products and understand chemical changes.

Additional info: These notes cover introductory topics in chemistry, including composition, formulas, and reactions, suitable for college-level study and exam preparation.