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Section 3.3—Density
Do you want high or low density in your airbag?
Definitions
How heavy is it for its size?
Density- the ratio of mass to volume of a sample
Lead = high density…small size is very heavyAir = low density…large sample has very little mass
Density
D =m
V
MassIn grams (g)
VolumeIn liters (L) or mL
DensityIn g/L or g/mL
Don’t try to cancel out the units…density has “2 units” – a mass unit over a volume unit!
Example 1—Solving for Density
Example:What is the density of a
sample with a mass of 2.50 g and a volume
of 1.7 mL?
Example 1—Solving for Density
V
mD
mL
gD
7.1
50.2
Example:What is the density of a
sample with a mass of 2.50 g and a volume
of 1.7 mL?
D = ?
m = 2.50 g
V = 1.7 mL
mLgD 5.1
Example 2—Solving for Mass
Example:What is the
mass of a 2.34 mL sample with
a density of 2.78 g/mL?
Example 2—Solving for Mass
V
mD
mL
mmLg
34.278.2
Example:What is the
mass of a 2.34 mL sample with
a density of 2.78 g/mL?
m = ?
V = 2.34 mL
D = 2.78 g/mL
gm 51.6
× 2.34 mL2.34 mL ×
Example 3—Solving for Volume
Example:A sample is
45.4 g and has a density of 0.87 g/mL. What is the
volume?
Example 3—Solving for Volume
V
mD
V
gmLg 4.45
87.0
Example:A sample is
45.4 g and has a density of 0.87 g/mL. What is the
volume?
m = 45.4 g
D = 0.87 g/mL
V = ?
mLV 52
× VV ×
gmLgV 4.4587.0
0.87 g/mL 0.87 g/mL
Graphing Density
volume
massSlope
12
12
xx
yySlope
If we make the y-axis mass and the x-axis volume then…
Volume (mL)
Mas
s (g
)
Density
Then the slope equals Density!
Floating
Objects float when they are less dense than the substance they are in!
Fewer particles in the same space = less dense
More particles in the same space = More dense
Let’s Practice 1
Example:If a 22.7 g
sample has a volume of 47.8 mL, will it float
in water?
Let’s Practice 1
V
mD
mL
gD
8.47
7.22
Example:If a 22.7 g
sample has a volume of 47.8 mL, will it float
in water?
m = 22.7 g
V = 47.8 mL
D = ?
mLgD 475.0
The density of water is 1 g/mL. This sample’s density is lower. It will float.
Let’s Practice 2
Example:What volume is a sample that is 27.5 g and has a density of 3.5
g/mL?
Let’s Practice 2
V
mD
V
gmLg 5.275.3
Example:What volume is a sample that is 27.5 g and has a density of 3.5
g/mL?
V = ?
m = 27.5 g
D = 3.5 g/mL
mLV 9.7
× VV ×
gmLgV 5.275.3
3.5 g/mL 3.5 g/mL