Empirical and Molecular Formulas

Empirical Formulas 

-Empirical formulas are the simplest formula of a compound 

-They show only the simplest ratios, not the actual number of atoms

-Molecular formulas give the actual number of atoms 

-To determine the empirical formula we need to know the ratio of each element 

Example: A sample of an unknown compound is found to contain 8.4 g of Carbon 2.1g of Hydrogen and 5.6g of Oxygen. Determine the empirical formula: 

Atom /     Mass/   Molar Mass/  Moles/  Mole - Smallest  Mole/ Ratio

C        /      8.4g/         12.0       / 0.7      /                 2                    /  2   >  C(2) 

H        /      2.1g/          1.0       /    2.1   /                  6                   /    6  >  H(6)

O        /      5.6g/         16.0      /    0.35 /                  1                    /    1 >  O (1)

C2 H6 O = Empirical Formula 

The simplest ratio may be decimals. For certain decimals you need to multiply by a certain number.

Decimal                 Multiplying Co-efficient

0.5                                         2

0.33 or 0.66                           3

0.25 or 0.75                           4 

0.2, 0.4, 0.6, 0.8                    5 

Molecular Formulas 

-If you know an empirical formula, to find the molecular formula you need the molar mass

Example: 

The empirical formula for a substance is CH2O

and its molar mass os 60.0g/mol. Determine the molecular formula 

Empirical               Molecular

CH2O  >>>x2>>>> C2H4O2

30.0 g/mol  >>>x2>> 60.0 g/mol

Jomar Delos Santos