Percentage Composition
147
meaning 6.023 x \Qa caterpillars. However, the number is seldom useful except in talking about molecules or atoms. The metric prefixes commonly are
used to give such units as millimoles, nanomoles, or picomoles.
PROBLEM:
How many molecules are there in 20.0 g of benzene, C 6 H 6 ?
SOLUTION:
First find how many moles of C 6 H 6 there are in 20.0 g, then use Avogadro's
number to find the number of molecules.
Mole weight of C (i H (i = (6 x 12.0 g/mole) + (6 x 1.0 g/mole) = 78.0 g/mole
Moles of C 6 H (i = —' '
g , = 0.256 moles
78.0 g/mole
Number of molecules = (0.256 moles)(6.023 x 10:i molecules/mole)
= 1.54 x I0
23 molecules
PERCENTAGE COMPOSITION
A chemical formula may be used to compute the percentage composition of a
compound; that is, the percentage by weight of each type of atom in the compound.
PROBLEM:
Calculate the percentages of oxygen and hydrogen in water, H 2 O.
SOLUTION:
The formula shows that 1 mole of H 2 O contains 2 moles of H and 1 mole of O. The
mole weight is (2 x 1.0 g/mole) ^ ( 1 x 16.0 g/mole) = 18.0 g/mole. This calculation
shows that there are 2.0 g H and 16.0 g O in 18.0 g H 2 O. Therefore,
O = '
6 '°
e x 100 = 88.9%
18.0 g
PROBLEM:
Compute the percentages of K, Fe, C, N, and H 2 O in K,Fe(CN) (i -3H 2 O crystals.