Example1
A compound is 75% carbon and 25% hydrogen.
What is the emperical formula?
Element: Carbon Hydrogen
%Element: 75% 25%
Mass (g): 75 25
Divide Ar for
each element: 75/12=6.25 25/1=25
Amount divide by
smallest amount: 6.25/6.25 25/6.25
Ratio of amount: 1 4
Formula: CH^4
Emperical Formula is the simplest ratio of atoms present in a molecule.
To find out the empercial formula for a compound, you must find the amount of each element and then calculate the simplest whole number ratio of the amounts.
Example 2
Calclate the empercial formula of a compound made by combining 1.92g of magnesium with 5.68g of chlorine.
Element: Mg Cl
Mass: 1.92 5.68
Molar Mass: 24 35.5
Divide mass by
their molar mass: 1.92/24=0.8 5.68/35.5=0.16
Amount divide by
smallest amount: 0.8/0.8 0.16/0.16
Ratio: 1 2
Formula: MgCl^2
Finding the emperial formula
Warning! :
Emperical Formula are not the same thing as the REAL Molecular Formula - you've got to cancel out the numbers (simplifying).
From emperical to final formula
Ionic Compound (formula)
The diagram shows the simplest ratio of a structure of Lithium CHloride. Its ratio is 1:1 in this compound with an emp formula of LiCl.
This shows that Ionic Compounds has the same ratio as its emperical formula.
Molecular Compound (formula)
A molecular formula is the same as or a multiple of the
empirical formula, and is based on the actual number of atoms
of each type in the compound. This gives us more information on the compound.
Calculation to find molecular formulas:
n = Mr / Total of Emperical Formula than multiplying the number of atoms in the emperical formula by n.
Example: -Emp Formula: CH^2
-Mr : 42
-Molecular: ??
?? = 42 / [12+2(1)]
=3
CH^2 x 3 = C^3H^6 (Molecular Formula)
