Chemical Composition
Chapter 6
Counting by Massing

If we know the mass of one object, we can
figure out how many there are by taking the
total mass.
Example: If each barrel holds 55 gal of oil, how
many barrels were used to fill a 22,550 gal tank?

22,550 gal
410 barrels
Apples have an average mass of 350. g each. How
many apples are in a 16.5 kg crate?
16.5 kg  1000 g  1 apple  47.1 apples
1 kg
350. g
OR 47 apples
Atomic Masses

Atomic mass is described in terms of
Atomic Mass Units (AMU)

1 AMU = 1.66 x 10-24 g
OR 0.00000000000000000000000166 g
Atomic Masses on Periodic Tables are an average
of all of the isotopes
 Average Lithium (Li) atom  6.94 AMU
 Average Tungsten (W) atom  183.94 AMU

Examples

What is the mass of 2,100 atoms of Al?
2100 atoms  26.98 AMU  57,000 AMU
1 atom Al

How many atoms of Nickel would it take to have
a mass of 19,190 AMU?
1 atom Ni
19,190 AMU 
 326.9 atoms of Ni
58.71 AMU