Calculate the empirical and molecular formula of an organic compound whose percentage composition is C=70.54%, H=5.87% and O=23.52%

Given:

Molecular weight of compound=136 g.

Percentage composition is C=70.54%, H=5.87% and O=23.52%

Step-1: Calculate the empirical formula

  • Empirical formula: Empirical formula is the simplest formula of the compound which gives the simplest whole number ratio of the atoms of the various elements present in one molecule of the compound.
Element% Of ElementAtomic mass (g)Atomic ratioSimplest ratio
Carbon (C)70.541270.5412=5.875.871.47=3.99≈4
Hydrogen (H)5.8715.871=5.875.871.47=3.99≈4
Oxygen (O)23.521623.5216=1.471.471.47=1

As the simplest ratio of Carbon is 4, Hydrogen is 4 and Oxygen is 1, therefore the empirical formula comes out to be C4H4O.

Step-2: Calculate the empirical and molecular mass

Empirical formula weight: Empirical formula weight is obtained by the addition of the atomic weight of the various atoms present in the empirical formula.

Atomic weight of Carbon= 12 g

Atomic weight of Hydrogen= 1 g

Atomic weight of Oxygen= 16 g

Empirical formula weight= 4(Atomic weight of Carbon) + 4(Atomic weight of Hydrogen) + Atomic weight of Oxygen

Empirical formula weight= 4(12) + 4(1) + 16= 68 g

Molecular formula weight: Molecular formula weight is obtained by the addition of the atomic weight of the various atoms present in the molecular formula.

Molecular weight =136 g (given)

Step-3: Calculate the molecular formula

  • Molecular formula: Molecular formula is the actual formula of the compound which gives the actual number of the atoms present in the molecule of the compound.
  • Relationship between empirical formula and molecular formula is given as follows: Molecular formula = n × Empirical formula, where n is an integer such as 1,2,3, etc.
  • n=MolecularweightofthecompoundEmpiricalformulamass
  • n=13668=2

As stated above, Molecular formula = n× Empirical formula, and empirical formula is C4H4O.

Therefore, the molecular formula is (C4H4O)2 or C8H8O2.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.