06 Worked examples
Worked Example 1: Systematic Chemical Formula Derivation and Atom Counting
Problem Statement: Derive the chemical formula for Calcium Phosphate and calculate: (1) The number of different chemical elements involved; (2) The total number of individual atoms present in one formula unit.
Detailed Solution & Derivation: Step 1: Write chemical symbols: Calcium is Ca, Phosphate radical is PO₄. Step 2: Determine valencies: Calcium is divalent (valency 2), Phosphate is trivalent (valency 3). Step 3: Crisscross valencies: The valency 3 of phosphate goes to calcium (Ca₃), and the valency 2 of calcium goes to the whole phosphate group in parentheses: (PO₄)₂. Step 4: Formula: Ca₃(PO₄)₂. Step 5: Element count: 3 distinct elements (Calcium Ca, Phosphorus P, Oxygen O). Step 6: Total atoms count: 3 Ca atoms + (2 × 1) P atoms + (2 × 4) O atoms = 3 + 2 + 8 = 13 atoms.
Worked Example 2: Identifying Compound Families from Chemical Formulas
Problem Statement: Classify the following substances into Acids, Alkalis, Oxides, or Salts: (a) H₂SO₄, (b) KOH, (c) Fe₂O₃, (d) NaNO₃, (e) SO₃, (f) NH₄Cl.
- Detailed Solution & Derivation: (a) H₂SO₄ is an Acid because its formula starts with hydrogen (H⁺).
- (b) KOH is an Alkali/Base because it terminates with the hydroxide radical (OH⁻).
- (c) Fe₂O₃ is a Basic Metallic Oxide (Iron(III) oxide).
- (d) NaNO₃ is a soluble Salt composed of Na⁺ cation and NO₃⁻ nitrate radical.
- (e) SO₃ is an Acidic Non-metallic Oxide (Sulfur trioxide).
- (f) NH₄Cl is a Salt composed of positive ammonium radical NH₄⁺ and negative chloride ion Cl⁻.