Calculating Molecular Mass Tutorial


Water Molecule

Water molecule with bond length and bond angle labelled. Credit: Todd Helmenstine

Molecular mass is the mass of one molecule.

Calculating molecular mass requires you to know what the atoms are that make up the molecule. Once you know the atomic makeup of the molecule, you can use the periodic table to find the mass of each atom and add them together.

Take water for example. Water has a molecular formula of H2O. This means one water molecule is made up of two hydrogen atoms and one oxygen atom.

The periodic table shows the atomic mass of hydrogen is 1.0008 amu and the atomic mass of oxygen is 15.999 amu. The molecular mass of water will be 2 masses of hydrogen plus the mass of oxygen.

molecular mass of H2O = (2 x 1.008 amu) + (15.999 amu)
molecular mass of H2O = 2.016 amu + 15.999 amu
molecular mass of H2O = 18.015 amu

The molecular mass of water is 18.015 amu.
Remember that one amu is equivalent to 1 gram/mole. This is how you can find the molecular weight of a molecule. This means one mole of water weighs 18.015 grams.

Example:
What is the molecular mass of sulfuric acid (H2SO4)?

Solution
Sulfuric acid is made up of 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms. So the molecular mass is:

Molecular mass of H2SO4 = (2 x mass of hydrogen) + (1 x mass of sulfur) + (4 x mass of oxygen)

From the periodic table, we see

mass of H = 1.008 amu
mass of S = 32.066 amu
mass of O = 15.999 amu

Plug these values into the formula.

Molecular mass of H2SO4 = (2 x 1.008 amu) + (1 x 32.066 amu) + (4 x 15.999 amu)
Molecular mass of H2SO4 = 2.016 amu + 32.066 amu + 63.996 amu
Molecular mass of H2SO4 = 98.078 amu

Answer:
The molecular mass of sulfuric acid is equal to 98.078 amu or 98.078 grams/mole.

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