Chemistry Reference
In-Depth Information
Oxygen-depleted
air
Figure 3.1
Illustration of the
Law of Conservation of Mass
In an eighteenth-century experiment,
phosphorus in air is ignited by sunlight
focused with a magnifying glass. The
phosphorus reacts with the oxygen
present in the air to produce an oxide
of phosphorus, which dissolves in the
water. The mass of the system after the
reaction is the same as it was before,
but the volume of gas trapped in the
bell jar has obviously been reduced.
Air
Dish
Dish
Phosphorus
Water
Water
Balance
pan
Before reaction: phosphorus
in air trapped in bell jar
After reaction: an oxide of
phosphorus dissolved in water
EXAMPLE 3.1
A 4.33-g sample of dinitrogen monoxide is composed of 63.65% nitrogen and
36.35% oxygen by mass. What is the percent composition of a 14.9-g sample
of dinitrogen monoxide?
Solution
The 14.9-g sample is 63.65% nitrogen and 36.35% oxygen also.
All
samples of
dinitrogen monoxide have the same percent composition, as required by the law
of definite proportions.
Practice Problem 3.1
Nitrogen monoxide has a percent composition
of 46.68% nitrogen and 53.32% oxygen. What possible percentages of nitrogen
could be in a
mixture
of dinitrogen monoxide and nitrogen monoxide?
EXAMPLE 3.2
Calculate the mass of nitrogen in a 4.75-g sample of nitrogen monoxide, using
the percentages in Practice Problem 3.1.
Solution
a
46.68 g N
100.0 g NO
4.75 g NO
b
2.22 g N
Total
mass
Mass of
component
Percentage
From the
percent composition
Practice Problem 3.2
Calculate the mass of nitrogen monoxide that
contains 100.0 g of nitrogen.
The
law of multiple proportions
states that for two (or more) compounds
composed of the same elements, for a given mass of one of the elements, the
ratio of masses of any other element in the compounds is a small, whole-number
