How to find empirical and molecular formula.
Example.
The sample of compound, which consist of hydrogen and carbon only, contains 3.6 g of carbon and 0.7 g of hydrogen. Molecular mass of the compound is 86. What is molecular formula of this compound?
Data: Solution:m (C)=3.6 g 1) Formula of the compound looks like
m (H)=0.7g CxHy.
M (compound)=86 2) First you have to find an empirical formula. Subscripts x and y
in an empirical formula can be found
Molecular formula x = m (C)/atomic mass (C) = 3.6/12 =0.3
of the compound-? y = m (H)/atomic mass (H) = 0.7/1 =0.7
x and y are supposed to be in ratio to each other as integer
numbers.
Its obvious here that x:y = 3:7
(Usually you are supposed to divide both numbers by the smallest
one. For example, x = 0.3/0.3 =1; y = 0.7/0.3 = 2.3. However
you cannot round 2.3 because it will produce too big error. So
you have to multiply both numbers by common multiplier to
obtain integers. x = 1*3 = 3; y = 2.3*3 = 7.) Hence, an
empirical formula is C3H7.
3) To find molecular formula you have to find how many times
molecular mass of molecular formula is greater than molecular
mass of empirical formula.
M (compound)/M (C3H7) = 86/43 = 2.
Thus you have to increase each subscript two times and
molecular formula of the compound is C6H14.