1. Molecular Formulas
Chapter 7-4, p

2. Molecular Formula Defined as:
the actual formula of a molecular compound.

3. Molecular vs. Empirical Formulas
C2H4 (ethylene) and C3H6 (cyclopropane) have the same empirical formula (CH2) but are very different chemicals:
ethylene is a gas used to ripen fruit, like bananas; cyclopropane was used as an anesthetic, until it was discovered it’s explosive when mixed with O2

4. Determining a Molecular Formula from an Empirical Formula
The relationship between a compound’s empirical formula and its molecular formula is…
X (empirical formula) = molecular formula, where X is a whole number
Also…
X (empirical formula mass) = molecular formula, where X is a whole number

5. Example
What is the molecular formula of diborane if its formula mass is g/mol, and the empirical formula is BH3?
10.81 g/mol + 3(1.01 g/mol)=13.84 g/mol
(empirical formula mass)
X = g/mol = = 2
13.84 g/mol
2(BH3) = B2H6

6. You Try It!
P2O5 is the empirical formula of a compound. The molecular formula mass is g/mol. Find its molecular formula.

7. You Try It! Solution
P2O5 is the empirical formula of a compound. The molecular formula mass is g/mol. Find its molecular formula.
P2O5 = 2(30.97g/mol) + 5(16.00g/mol)
= g/mol
X = g/mol = = 2
g/mol
2(P2O5) = P4O10