## How to calculate molecular mass?

1) Determine the mass in 1 mole – also known as molar mass.
2) Divide the molar mass by Avogadro’s constant to get the mass in 1 molecule.

## Example: Molecular Mass of Water H2O

 ELEMENT ATOMIC MASS NUMBER TOTAL ATOMIC MASS hydrogen 1.00078 2 2.00156 g/mol oxygen 15.9949 1 15.9949 g/mol Total 18.0105 g/mol

## Molecular mass of water

= 18.0105 g/mol ÷ (6.0221 X 10^23) molecules/mol
= 2.9907 x10^(-23) g/molecule

## Molecular Mass of Water

### Expressed as a mole: 18.0105 g/mol Expressed as a single molecule: 2.9907 x10^(-23) g/molecule

If you see molecular mass expressed in AMU, don’t worry! AMU and g/mol are equivalent to each other.

## 3 thoughts on “What is Molecular Mass?”

1. George Davis says:

Beautiful presentation. Aren’t computers wonderful. When I taught chemistry, the best we had were calculators and overheads. You are so blessed. I’ll try to convey the way I approached the teaching of the mole. It would be easier to draw it than to type it, but I’ll try.
If you have an email I could send a written copy to you if you are interested in my approach.
dozen is 12; gross is 144; elom is 16. An object has a mass of 5 oz. What is the mass of an elom of them in pounds? 5 oz. x 1 lb. x 16 obj = 5 lbs.
1 obj 16 oz. 1 elom 1 elom
Since mole is 6.02 x 10 e 23 & there are 6.02 x 10 e 23 amu = 1 g, that’s why mass in amu is numerically equal to mass in grams { substitute amu for oz., grams for lb, and mole for elom in the above example. 16 was chosen for elom to numerically match 16 oz./lb So, 6.02 x 10 e23 was chosen to match 6.02 x 10 e 23 amu/g.}

2. Piet Surinx says:

The example of water: you speak about atomic weight, it must be atomic MASS!! (weight is a force: mass * gravity).
See you! Piet

1. Surfguppy says:

Thanks Piet for submitting your comment! I’ve changed the term “atomic weight” to “atomic mass” to be absolutely clear.

Please see “Molecular Weight, Atomic Weight, Weight vs. Mass” in Chemwiki