How to Calculate Molecular Weight

Step-by-Step Guide

1. 1

   Step 1: Count how many atoms of each element exist in the molecule.

   First, list each element present in the molecule.
   
   You may use the chemical symbol or write out the name of the element.
   
   Then, count the atoms according to the molecular formula.For methane, CH4, list carbon (C) and hydrogen (H).
   
   Methane comprises one atom of carbon and four atoms of hydrogen.
   
   For glucose, C6H12O6, list carbon (C), hydrogen (H), and oxygen (O).
   
   Glucose comprises six atoms of carbon, twelve atoms of hydrogen, and six atoms of oxygen.
2. 2

   Step 2: Find the relative atomic mass of each element in the molecule.

   Use a copy of the Periodic Table of Elements.
   
   The Periodic Table lists the atomic mass of each element below the chemical symbol. For example, oxygen has a relative atomic mass of
   15.9994 amu.The atomic mass of an element is roughly equal to the summed mass of the protons and neutrons that it contains.
   
   Note that the relative atomic mass you find in the periodic table is scaled: it accounts for all of the isotopes of the element in the proportions that they naturally occur., Multiply the atomic mass of each element by the number of atoms of that element: (Atomic Mass of Element) x (# of atoms of that element).
   
   Do this for each element in the molecule., Molecular weight = ∑((Atomic Mass of Element)n x (# of atoms of that element)n)Round the answer as necessary, using significant digits.
   
   Remember to use the proper units. amu is the old abbreviation for atomic mass units, but the "most correct" modern unit is a lower-case u.
3. 3

   Step 3: Calculate the total mass for each element in the molecule.

4. 4

   Step 4: Add up the mass of all the atoms to find the molecular weight.

Detailed Guide

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