EXAMPLE 1-10 Percentage
U.S. pennies made since 1982 consist of 97.6% zinc and 2.4% copper. The mass of a partic-
ular penny is measured to be 1.494 grams. How many grams of zinc does this penny contain?
Plan
From the percentage information given, we may write the required unit factor
Solution
__? g zinc1.494 g sample1.46 g zincThe number of significant figures in the result is limited by the three significant figures in
97.6%. Because the definition of percentage involves exactly100 parts, the number 100 is known
to an infinite number of significant figures.
You should now work Exercises 59 and 60.
DENSITY AND SPECIFIC GRAVITY
In science, we use many terms that involve combinations of different units. Such quanti-
ties may be thought of as unit factors that can be used to convert among these units. The
densityof a sample of matter is defined as the mass per unit volume:
density or DDensities may be used to distinguish between two substances or to assist in identifying a
particular substance. They are usually expressed as g/cm^3 or g/mL for liquids and solids
and as g/L for gases. These units can also be expressed as gcm^3 , gmL^1 , and
gL^1 , respectively. Densities of several substances are listed in Table 1-8.
EXAMPLE 1-11 Density, Mass, Volume
A 47.3-mL sample of ethyl alcohol (ethanol) has a mass of 37.32 g. What is its density?
Plan
We use the definition of density.
Solution
D0.789 g/mLYou should now work Exercise 36.
37.32 g
47.3 mLm
Vm
Vmass
volume1-11
97.6 g zinc
100 g sample97.6 g zinc
100 g sampleThe intensive property densityrelates
the two extensive properties: massand
volume.1-11 Density and Specific Gravity 31See the Saunders Interactive
General Chemistry CD-ROM,
Screen 1.8, Density.MassDensityVolumeMass
Volumeh(^66)
(^66)
(^66)
(^66)
(^6) g
Six materials with different densities.
The liquid layers are gasoline (top),
water (middle), and mercury (bottom).
A cork floats on gasoline. A piece of
oak wood sinks in gasoline but floats
on water. Brass sinks in water but
floats on mercury.