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