What is 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 on empirical formula:
A compound has 44.09 % iron and rest chlorine. Determine its emp 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 emp formula is FeCl2 so now to find out molecular formula,
use Molecular formula = n X Empiricalformula where n=1
hence Empirical formula = Molecular formula
Practice questions:
1. A binary oxide has 74.19 % sodium and rest oxygen. Determine its emp 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 emp 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 emp 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 emp 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 emp formula. If the molar mass of the compound is 180 g/mol, determine its Molecular formula as well.
Read Mole Concept
