How to calculate molecular mass?
- Determine the formula of the compound
- Use the periodic table, determine the atomic mass of each element present in the formula
- Multiply the atomic mass of each element by the number of atoms
- Finally, add the atomic masses
Example: How to find the molecular mass of water?
- First, determine the formula for water.
- The formula for water is H_{2}O.
- Use the periodic table to look up the atomic mass for the elements in the formula.
- You must then count how many atoms of each type are present using the formula.
ELEMENT | ATOMIC MASS | #ATOMS | TOTAL |
hydrogen | 1.008 amu | 2 | 2(1.008) = 2.016 amu |
oxygen | 16.00 amu | 1 | 1(16.00) = 16.00 amu |
Total the column
= 18.02 amu (rounded) |
Molecular mass can also be expressed in terms of moles (g/mol)
Substance | Formula | Molecular Mass | Molar Mass |
Water | H_{2}O | 18.02 amu | 18.02 g/mol |
Carbon dioxide | CO_{2} | 44.01 amu | 44.01 g/mol |
Oxygen | O_{2} | 32.00 amu | 32.00 g/mol |
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