Molecular Formula
Relate the empirical formula to the true molecular formula
Formula Relation
Molecular formula = (Empirical formula) × n
n = (molar mass of compound) / (empirical formula mass)
Steps
- Find the empirical formula from composition or ratios.
- Compute empirical formula mass (sum atomic masses).
- Measure or obtain compound\'s molar mass (g/mol).
- Compute n = (molar mass) / (empirical mass) and multiply subscripts by n.
Example
Given: Empirical formula CH2O, molar mass ≈ 180.16 g/mol.
1) Empirical mass of CH2O = 12.01 + 2(1.008) + 16.00 = 30.026 g/mol.
2) n = 180.16 / 30.026 ≈ 6.
3) Molecular formula = (CH2O) × 6 = C6H12O6.
Answer: C6H12O6 (glucose)
Common Pitfalls
Rounding n incorrectly
Use nearest whole number; if slightly off, multiply to eliminate fractions (e.g., ×2 or ×3).
Empirical mass errors
Sum atomic masses accurately; keep extra decimals until the end.
Misreading molar mass
Use the correct measured molar mass (from MS or other data) for the compound.
Non-integer n
If n ≈ 1.5, multiply empirical subscripts by 2 to get integers, then verify with molar mass.
FAQ
Can empirical and molecular formulas be the same?
Yes, when n = 1 (e.g., H2O).
What if molar mass is uncertain?
Compute n with the best estimate, then check if the resulting molecular mass matches the measured value.
How to handle hydrates?
Include waters of hydration in empirical and molecular formulas if measuring the hydrate.