Chemistry 11 : Empirical Formula

The empirical formula for a compound is it s chemical formula in lowest terms ⇒  Example -   H2O2            Empirical Formula       HO -    C4H8             Empirical Formula      CH2 -   C6H12O6      Empirical Formula     CH2O -   MgCl2             Empirical Formula      MgCl2

Note : Most empirical formulas for compounds are identitical to its molecular formula

Different compounds can have the same empirical formula Examples : C2H4 C3H6 C4H8 C10H20

You can determine the empirical formula using percentage composition data for a compound Set up the following table Element   Mass %    Mass in 100g    Molar mass (g/mol)    # of Moles (mol)   Mol Ratio C   79.9%    79.9    12.01    6.65mol    1 H   20.1%    20.1%    1.01    19.9    2.99 (3)

And fill in the green information which is the first 2 columns Column 3 is simply the mass % in grams Column 4 is the molar mass of each element (from the periodic table) Column 5 is calculated using columns 3 and 4 (n = m * 1/m) Column 6 is the mole ratio determined by diving each elements number of moles by the lowest number of moles

Example ⇒  What is the empirical formula of a compound containing 85.6% carbon and 14.4% Hydrogen N(c)= 85.6g * 1mol/12.01g  = 7.13mol N(h)=14.4g * 1mol/1.01g = 14.3mol Empirical Formula = CH2

For some compounds, the mole ratio doesn’t workout to a whole number

.5 multiply by 2 .33 .67 multiply by 3 .25 or .75 multiply by 4 .2 .4 .6 or .8 multiply by 5