Chemical Formula Calculation in Mole Concept

What is a Chemical Formula?

A chemical formula shows the types and numbers of atoms in a molecule or compound. It can be expressed as:

• Empirical Formula: Simplest ratio of elements (e.g., CH₂O)
• Molecular Formula: Actual number of atoms (e.g., C₆H₁₂O₆)

Role of Mole Concept

The mole concept helps convert mass to moles, find ratios, and determine formulas.

Steps to Calculate Formulas

1. Convert mass/percentage into moles using n = mass / molar mass
2. Divide all mole values by the smallest mole value
3. Write empirical formula using mole ratios
4. Calculate molecular formula if molar mass is given:
   • Find empirical formula mass
   • Calculate ratio = Molar Mass / Empirical Mass
   • Multiply subscripts

Example 1: Empirical Formula

40% C, 6.7% H, 53.3% O

C: 40 ÷ 12 = 3.33 mol
H: 6.7 ÷ 1 = 6.7 mol
O: 53.3 ÷ 16 = 3.33 mol
Ratios = C:1, H:2, O:1 → CH2O

Example 2: Molecular Formula

Empirical formula: CH2O, Molar mass = 180 g/mol

Empirical mass = 30 g/mol → 180 ÷ 30 = 6
Molecular formula = C6H12O6

Multiple Choice Questions (MCQs)

1. What does the empirical formula represent?
   A. Actual number of atoms
   B. Simplest ratio of atoms ✅
   C. Molar mass
   D. Molecular structure
   Explanation: The empirical formula shows the simplest whole-number ratio of elements.
2. If empirical mass is 30 g/mol and molar mass is 90 g/mol, what is the molecular formula ratio?
   A. 2
   B. 3 ✅
   C. 1.5
   D. 0.3
   Explanation: Ratio = 90 ÷ 30 = 3

✅ True or False

• The molecular formula is always a multiple of the empirical formula. — True ✅
  Explanation: It is derived by multiplying the empirical formula by a whole number ratio.
• Mole concept is not useful in determining chemical formulas. — False ❌
  Explanation: It is essential in converting mass into mole ratios for formula calculation.

Quick Quiz

Q: A compound contains 24 g of C, 4 g of H, and 32 g of O. What is its empirical formula?

Answer:

• C: 24 ÷ 12 = 2 mol
• H: 4 ÷ 1 = 4 mol
• O: 32 ÷ 16 = 2 mol

Ratio: C:2, H:4, O:2 → Divide all by 2 → CH2O

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🔗 External Reference: Wikipedia: Chemical Formula

Mastering formula calculation helps in understanding chemical compositions and reactions accurately.

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