Molecular and Empirical Formula

Molecular formula and Empirical formula of compounds.

 

Empirical formula and molecular formula EF and MF

Empirical Formula: E.F. The empirical formula is determined experimentally and gives us the simplest ration in which the elements are combined.

Molecular Formula: M.F. Gives us the actual ratio in which the elements are combined in a compound
Find the value of n from the data provided in the following table.

EFM EF Name MF MM n = MF/EF
18 H2O Water H2O 18
13 CH Benzene C6H6 78
30 CH2O Glucose C6H12O6 180
17 HO Hydrogen Peroxide H2O­2 34
44 CO2 Carbondioxide CO2 44
29 C2H5 Butane C4H10 58
14 CH2 Ethene C2H4 28
13 CH Ethyne C2H2 26
98 H2SO4 Sulfuric acid H2SO4 98

What is the relation between EF and MF or what is the relation of MF to EF

clip_image002

clip_image004

Empirical formula problems:

  1. What is the empirical formula for a compound whose percentage composition is 21.6% sodium, 33.3% chlorine and 45.1 % oxygen?
Na Cl O
% 21.6 33.3 45.1
Moles 0.940 0.939 2.82
Simplest ratio 1 1 3
EF Na Cl O3

The empirical formula is NaClO3

  1. Compound Y contains 69.6 % carbon, 6.86 % hydrogen and the remaining oxygen. Determine the empirical formula of compound Y. [Ans: C4H9O2 (C2H4.5O1)[
  2. A sample of a substance is determined to be composed of 0.89 grams of potassium, 1.18 grams of chromium, and 1.27 grams of oxygen. Calculate the empirical formula of this substance. [Ans. K2Cr2O7[

Molecular formula problems:

  1. The empirical formula of butane is C2H5. The molar mass of butane is determined to be 58 g/mol. What is the molecular formula of this substance?
  • Compound X has a molar mass of 240.28 g/mol. Its % composition is 75% carbon, 5.05 % hydrogen and the remaining oxygen. What is the compounds molecular formula? [Ans: (C5H4O)3 = C15H12O3 EF mass = 80]

  • A 2.78 g hydrate of iron (II) sulphate FeSO4.xH2O was heated to remove all the water of hydration. The mass of the anhydrous iron (II) sulphate was 1.52g. Calculate the number of water molecules associated with each molecule of FeSO4.[Ans 7]