SECTION 1.4 Units of Measurement 19
Syringes, burets, and pipets deliver amounts of liquids with more precision than
graduated cylinders. Volumetric flasks are used to contain specific volumes of liquid.
GIVE IT SOME THOUGHT
Which of the following quantities represents volume measurements:
? How do you know?
Density
Density is defined as the amount of mass in a unit volume of a substance:
[1.3]
The densities of solids and liquids are commonly expressed in either grams per
cubic centimeter or grams per milliliter . The densities of some com-
mon substances are listed in 씰TABLE 1.6.It is no coincidence that the density of water
is ; the gram was originally defined as the mass of 1 mL of water at a specific
temperature. Because most substances change volume when they are heated or cooled,
densities are temperature dependent, and so temperature should be specified when
reporting densities. If no temperature is reported, we assume 25 °C, close to normal
room temperature.
The terms density and weight are sometimes confused. A person who says that iron
weighs more than air generally means that iron has a higher density than air—1 kg of air
has the same mass as 1 kg of iron, but the iron occupies a smaller volume, thereby giving
it a higher density. If we combine two liquids that do not mix, the less dense liquid will
float on the denser liquid.
1.00 g>mL
(g>mL)(g>cm3)
Density =mass
volume
15m2; 2.5 *102 m3; 5.77 L/s
TABLE 1.6 •Densities of Selected
Substances at 25 °C
Substance Density (g/cm3)
Air 0.001
Balsa wood 0.16
Ethanol 0.79
Wat er 1.00
Ethylene glycol 1.09
Tab le su ga r 1.59
Table salt 2.16
Iron 7.9
Gold 19.32
SAMPLE EXERCISE 1.4 Determining Density and Using Density to Determine Volume or Mass
(a) Calculate the density of mercury if occupies a volume of .
(b) Calculate the volume of 65.0 g of liquid methanol (wood alcohol) if its density is .
(c) What is the mass in grams of a cube of gold if the length of the cube is 2.00 cm?
SOLUTION
(density =19.32 g>cm3)
0.791g>mL
7.36 cm3
1.00 *102 g
(a) We are given mass and volume, so Equation 1.3 yields Density =mass
volume
=1.00 *102 g
7.36 cm3=13.6 g>cm3
(b) Solving Equation 1.3 for volume and then using the given mass
and density gives Volume =mass
density
=65.0 g
0.791 g>mL
=82.2 mL
(c) We can calculate the mass from the volume of the cube and its density.
The volume of a cube is given by its length cubed: Volume =(2.00 cm)3=(2.00)3 cm3=8.00 cm3
Solving Equation 1.3 for mass and substituting the volume and density
of the cube, we have Mass =volume *density =(8.00 cm3)(19.32 g>cm3)=155 g
PRACTICE EXERCISE
(a) Calculate the density of a 374.5-g sample of copper if it has a volume of . (b) A student needs
15.0 g of ethanol for an experiment. If the density of ethanol is , how many milliliters of ethanol
are needed? (c) What is the mass, in grams, of 25.0 mL of mercury ?
Answers: (a) ,(b) 19.0 mL, (c) 340 g8.96 g>cm3
(density =13.6g>mL)
0.789g>mL
41.8 cm3
