What is Empirical formula? How to determine it?

What is Empirical formula? How to determine it?and Determining Molecular formula:

Empirical formula:  Chemical Formula with simplest ratio of atoms of a compound.

Molecular formula: Chemical Formula with definite number of atoms of a compound.

Example:  

C₆H₆ is the molecular formula of benzene.

CH is the simplest ratio of atoms of C₆H₆

Molecular formula = n X Empirical formula 

where n=Molecular formula mass/Empirical formula mass

  molecular formula mass of C₆H₆ is 78 g/mol

Empirical formula mass of CH is 13 g/mol

n= 78/13= 6

Molecular formula = n X Empirical formula = 6 X CH= C₆H₆

Step 1: Write the element symbols along with % given. Consider % as mass in gram.

Step 2: Divide the given mass of each element by relative atomic mass of that element.

( mole=mass/molar mass) so you get moles of each element here.

Step 3: Divide the moles of each element by the smallest moles obtained in step 2.

Step 4: Use the obtained values as number of atoms per element in empirical formula.

Note : if you did not get the whole number values, do not simply round off the values but multiply by suitable integers to make the value whole number. Example if you get 2.5, do not round simply to 3 but multiply 2.5 by 2 to get 5 which is a whole number.

Worked Problem:   A compound has 44.09 % iron and rest chlorine. Determine its empirical formula.  If the molar mass of the compound is 127 g/mol, determine its Molecular formula as well.

Solution: 

Step1	Iron(Fe)=44.09	Chlorine(Cl)=100-44.09=55.91
Step2	44.09/56	55.91/35.5
Step3	0.79/0.79=1	1.57/0.79= 1.98=2
Step4	1	2

So the empirical formula is FeCl₂  so now to find out molecular formula, 

use Molecular formula = n X Empirical formula where n=1

hence Empirical formula = Molecular formula

Practice questions:  

1.       A binary oxide has 74.19 % sodium and rest oxygen. Determine its empirical formula.  If the molar mass of the compound is 62 g/mol, determine its Molecular formula as well.

2.       A compound has 43.39 % sodium, 11.32% carbon  and rest oxygen. Determine its empirical formula.  If the molar mass of the compound is 106 g/mol, determine its Molecular formula as well.

3.       A compound has 34.46 % iron and rest chlorine. Determine its empirical formula.  If the molar mass of the compound is 162.5 g/mol, determine its Molecular formula as well.

4.       A binary oxide has 43.66 % phosphorus and rest oxygen. Determine its empirical formula.  If the molar mass of the compound is 284 g/mol, determine its Molecular formula as well.

5.       A compound has 40 % carbon, 6.6% hydrogen  and rest oxygen. Determine its empirical formula.  If the molar mass of the compound is 180 g/mol, determine its Molecular formula as well.