Empirical Formula & Compound Nomenclature

Naming of Ionic Compounds

• Binary ionic compounds ➔ metal name + non-metal root + “-ide”
  • Examples: $\text{NaOH}$ = sodium hydroxide; $\text{KI}$ = potassium iodide
• Compounds with polyatomic ions ➔ keep ion name unchanged
  • $\text{CaSO<em>4}$ = calcium sulfate • $\text{(NH</em>4)<em>2SO</em>4}$ = ammonium sulfate
• Formula writing tips
  • Total positive charge = total negative charge
  • Reduce subscripts to lowest whole-number ratio

Percent Composition

• Definition: % by mass of each element in a compound
• Formula: $\%\,\text{element}= \frac{n\,(\text{molar mass of element})}{\text{molar mass of compound}}\times100$
• CO$_2$ example
  • $\%\,\text{C}=\frac{1(12.01)}{44.01}\times100=27.29\%$
  • $\%\,\text{O}=\frac{2(16.00)}{44.01}\times100=72.71\%$

Empirical Formula

• Shows simplest whole-number mole ratio of elements
• May differ from molecular formula (actual atom count)
• Can be derived from percent composition data

Steps to Determine Empirical Formula

1. Assume $100\,\text{g}$ sample ➔ % → g
2. Convert g → mol using molar masses
3. Divide each mol value by the smallest mol value
4. If any ratio is non-integer, multiply all ratios by the smallest integer that makes each whole
5. Write formula with these whole-number subscripts

• Memory rhyme: “Percent to mass, mass to mole, divide by small, multiply till whole”

Example Summaries

• 7.81 % C, 92.19 % Cl
– Moles: C 0.650, Cl 2.601 ➔ divide ➔ 1 : 4 ➔ $\text{CCl}<em>4$ • 52.9 % Al, 47.1 % O – Moles: Al 1.959, O 2.944 ➔ ratio ≈ 2 : 3 ➔ $\text{Al}</em>2\text{O}_3$
• Practice problems to try:
– 50.05 % S, 49.95 % O ➔ ?
– 43.64 % P, 56.36 % O ➔ ?

Significance & Applications

• Nutrition labels: relate mass % of elements (e.g., Na, Fe) to dietary limits
• Skincare: adjust elemental ratios for desired pH/viscosity (e.g., ZnO formulations)
• Pharmaceuticals: confirms identity & purity of active ingredients ensuring dosage accuracy
• Agriculture: guides N–P–K ratios in fertilizers for optimal plant growth