The molecular weight of a compound is the weight of 1 mole of molecules of that compound. To find the molecular weight of a compound, look up the atomic weights of the atoms in the molecule and add them up.

For example, to compute the molecular weight of baking soda, NaHCO₃, we look up the atomic weights of the atoms, then add them together

Atom	Atomic weight (g/mole)	Number in molecule	Total weight
Na	22.99	1	22.99
H	1.008	1	1.008
C	12.01	1	12.01
O	16.00	3	48.00
NaHCO3			84.008

One mole of NaHCO³ contains 6.022*10²³ molecules of NaHCO₃ and weighs 84.008 g/mole.

The molecular weight is used to convert from grams to moles and vice versa. To convert from a weight of a substance to the number of atoms (or moles) just divide the weight by the molecular weight. To convert from moles to grams, multiply the number of moles by the molecular weight.

For example, to compute the weight of 0.330 moles of NaHCO₃, multiply the number of moles by the molecular weight.

0.330 moles * 84.008 grams/1 mole = 27.7 grams

For another example, to compute the number of moles of NaHCO₃ in 150 grams of NaHCO₃, work out the molecular weight (84.008 g/mole) and just divide

150 grams * 1 mole/84.008 grams = 1.79 moles of NaHCO₃

Example 1: What is the molecular weight of vinyl chloride, C₂H₃Cl?

Solution 1: Find out the atomic weights of each species and add them together.

Atom	Atomic weight (g/mole)	Number in molecule	Total weight
C	12.01	2	24.02
H	1.008	3	3.024
Cl	35.45	1	35.45
C2H3Cl			62.494

1 mole of C₂H₃Cl weighs 62.494 grams

Example 2: How many moles of C₂H₃Cl are there in 20.0 grams of C₂H₃Cl?

Solution 2: Simply use the molecular weight to convert from grams to moles

20.0 grams * 1 mole/62.494 grams = 0.320 moles of C₂H₃Cl

Example 3: How much does 4.50 moles of C₂H₃Cl weigh?

Solution 3: Use the molecular weight again to convert from moles to grams

4.50 moles * 62.494 grams/1mole = 281 grams

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