Carnegie Mellon University

Sample Molecular Weight Calculation

  • The chemical formula for water is H2O, which means this molecule has 3 atoms: 2 of hydrogen (H) and 1 oxygen (O) atom. Using the periodic table of the elements to find atomic weights, we find that hydrogen has an atomic weight of 1, and oxygen's is 16. In order to calculate the molecular weight of one water molecule, we add the contributions from each atom; that is, 2(1) + 1(16) = 18 grams/mole.
  • The chemical formula for an ethylene monomer is -(CH2-CH2)-. It has a total of 6 atoms: 2 carbon (C) atoms and 4 hydrogen (H) atoms. The atomic weight of carbon is 12; and that of hydrogen is 1, so one mer of ethylene has a weight of 2(12) + 4(1) = 28. We combine (react) many mers of ethylene together to form a polyethylene chain. One thousand mers connected together would add up to a weight of 28,000 grams/mole and would have 6,000 atoms.

A mole is the standard method in chemistry for communicating how much of a substance is present. A mole is the unit that measures the amount of a substance. One mole of 'something' contains 6.022 x 1023 entities. For example, in one mole of a chemical compound there are 6.022 x 1023 molecules.

Let us calculate the molecular weight of some common compounds. Use the table below to find the atomic weight of each atom (element), or refer to a Periodic Table of the Elements. Hint: Count the number of atoms of each element, and then multiply that number by the element's atomic weight.

H (Hydrogen) 1
C (Carbon) 12
N (Nitrogen) 14
O (Oxygen) 16
Na (Sodium) 23
Cl (Chlorine) 35

  1. Table salt - NaCl
    Answer: 23(1)+35(1)=58
  2. Ammonia - NH3
    Answer: 1(3)+14(1)=3+14=17
  3. Vinegar (acetic acid) - CH3COOH
    Answer: 1(4)+12(2)+16(2)=4+24+32=60