## 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 H_{2}O

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 can also be expressed as a Mole (g/mol)

## Molecular Mass of Water

## Summary

## 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.

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.}

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

See you! Piet

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