Emperical Formula and Molecular Formula

Emperical formula:

• The formula which represents in simple ratio of various atoms present in the compound.

Molecular formula:

• The formula which represents exact number of atoms present in the compound.

Few formulaes are relating here to discuss the realtion between emperical formula and molecular formula.

• Sometimes we will have the data about the compound in the form of percentage of each element.By that time we need to follow some of the below given rules to write the molecular formula of the compound.

1. Conversion of mass percentage into grams
2. Conversion of grams into number of moles of each element by dividing the given mass with its atomic mass respectively.
3. Divide each of mole values obtained by the smallest number among all elements.
4. Then we will get the simplest ratio of the elements.so that we can write the emperical formula.
5. Writing of molecular formula based on the above formulas.

Let us see one example by following the above rules.

Hydrogen – 4.07%    , carbon – 24.27%   ,  chlorine  – 71.65%

Hence we can say the emperical formula as CH₂Cl.

Emperical formula mass is 12+2+35.5 = 49.5

If we consider the molecular mass of the compound is 98.96gm.,then to find the molecular formula of the compound we should use the 5^th rule.