How can you calculate molecular weight?

1 Answer


You will need to use the chemical formula of the compound and the average atomic masses from the periodic table to calculate molecular mass .


Let's look at the example of water. I'll round average atomic masses to two places beyond the decimal. You should adjust your rounding based on your teacher's preferences.

#H_2O# = two atoms of H and one atom O in a single molecule

H = 1.01 (use the average atomic mass from the periodic table)
O = 16.00

To find molecular weight:
H = 1.01 x 2 = 2.02 (have to multiply by two since water has two hydrogen atoms)
O = 16.00 x 1 = 16.00

The total molecular weight is 18.02 - this was found by adding the total mass from H with the total mass from O.

This will also be the molar mass of water

Here are a couple more examples:

Hope this helps