Empirical formula is the formula of a chemical compound represented as the smallest whole number ratio of the subscripts of the elements in the compound. Particularly for big ionic compounds or compounds that possess complex and network structure, such as diamond, there is no distinctive molecule. Hence, they cannot be represented with their molecular formula. For such compounds, empirical formula is useful. Empirical formula is also known as stoichiometric formula. For example, CH3COOH has two carbon atoms, four hydrogen atoms, and two oxygen atoms. Hence, it can be expressed as C2H4O2 and reduced to CH2O.
Significance
Empirical formula offers useful information regarding the names of all the elements occurring in a compound, provides the simplest whole- number ratio between atoms of all elements occurring in the compound and expresses mass of the substance representative of the formula mass of the substance.
Difference and relation between Molecular Formula and Empirical Formula
Empirical formula is indicative of the reduced ratio of elements in the compound, whereas the molecular formula is representative of the actual number of atoms of every element in the compound. IF the molecular formula cannot be simplified further, the empirical formula is equal to the molecular formula .
If a covalent compound has a molecular formula C4H4, which is, four carbon atoms and four hydrogen atoms in the molecule. The smallest whole number ratio between the carbon and hydrogen atoms is 1:1 (4:4 simplified as 1:1).
Examples
Sodium chloride in its crystalline form occurs as a three dimensional structure consisting of sodium (Na+) and chloride Cl- ions . Hence, sodium chloride may be expressed as (Na+Cl-) n, where n is a large number. The real count of Na+ and Cl- ions in a sample of sodium chloride is dependent upon the size of the sample. However, irrespective of the size of the sample, the ratio between the number of Na+ ions and Cl- ions is always 1:1. Hence, the simplest formula or the empirical formula (or stoichiometric formula) of sodium chloride is NaCl.
Derivation of Empirical and Molecular Formula from % composition .
First step in deriving empirical formula is to determine percent Composition, which is equal to (mass of the elements/ mass of the compound) X 100. The percent composition is then used to determine the mass composition in moles, and then ultimately the smallest whole number mole ratio of atoms.
Example 1
For example, if a compound contains 72% Cl, 24% C and 4 % H, the mass of each element in 100g substance would be 72, 24 and 4g respectively. Molar mass is then employed to calculate the number of moles of each element in the substance, which is 72g* (1mole Cl/35.5gCl) = 2 moles, 24gC* (1mole C/12gC) = 2 moles, and 4 g H*(1mole H/1gH) = 4 moles. The number of moles is then divided by the smallest number of moles, i.e., Cl = 2/2 =1, C = 2/2 =1, H = 4/2 =2. Hence, the empirical formula is CH2Cl.
Example 2
If a compound comprises of 50.05 % sulphur (S) and 49.95 % oxygen (O) by weight and given the molecular weight of the compound is 64.07 g/mol, empirical formula can be deduced as follows :
100 g of the compound will contain 50.05 g of S and 49.95 g of O, equal to 50.05g/32.066g/mol =1.5608 moles of S and 49.95 g / 16.00 g/mol = 3.1212 moles of O. These values are then divided by the lowest, obtaining smallest whole-number ratio. i.e. S = 1 and O = 2 and hence the empirical formula is S02.
Works Cited
Calculate empirical formula when given percent composition data. n.d. http://www.chemteam.info/Mole/Emp-formula-given-percent-comp.html. 11 March 2016.
chemistry.about.com. 27 November 2014. 11 March 2016.
Meyers, Amy. http://study.com/academy/lesson/calculating-percent-composition-and-determining-empirical-formulas.html. 2016. 11 March 2016.
