3.3: Mass Composition

FORMULA WEIGHT

Weight (mass) of a compound (Molecular/Ionic) in atomic mass units -amu-

Example:

What is the molar mass of aluminum hydroxide? Al(OH)3

Step 1: Obtain the amount of each and multiply but the molar mass of each element:

1 Aluminum = 1 x 26.98 amu = 26.98

3 Oxygen  =   3 x 16.00 amu = 48.00

3 Hydrogen = 3 x 1.01   amu = 3.03

Step 2: Add all the obtained numbers:

26.98 + 48.00 + 3.03 = 78.01 amu

PERCENT COMPOSITION BY MASS

Percent of each element in a given compound.

Example:

What is the percent of Hydrogen in water, H2O?

Step1: Find the formula weight (molar mass) of each element:


2 Hydrogen = 2 x 1.01  amu = 2.02
1 Oxygen    = 1 x 16.00 amu = 16.00

Step 2: Use formula