Density Calculations
How do you calculate the density of a
substance?
Covered in This Lesson
• This lesson will demonstrate how to calculate
density to the correct number of significant
figures(sig figs) using experimental
measurements.
– When calculating density we measure the
mass and the volume of the sample.
– The volume can be measured using one of
several methods.
Density Formual
• When calculating density you measure the
– mass of the sample using a balance
– volume of the sample from one of the
methods
– apply the formula: mass/volume = density
or
m =
V
d
Methods: Volume
• There are three methods to measure the
volume of a substance. The method used is
determined by the type of substance.
• Methods for the;
1. volume of liquid
2. volume of regular solid
3. volume of irregular solid.
Volume of a Liquid
• If the
substance is
a liquid,
measure it
directly in a
graduated
cylinder.
Examples 1
• If a liquid sample has a mass of 4.83 g and a
volume of 2.7 mL what is its density.
4.83 g = 1.788889
2.7 mL
Because 2.7 has 2 sig figs
the final answer is 1.8 g/mL
Volume of a Regular Solid
• If you have a regular solid like a cylinder,
cube, or rectangular solid measure the
dimensions needed and apply the
volume formula.
Examples 2
• If cubic solid sample has a mass of 40.53 g and
the side is 2.55 cm, what is its density.
V = s3 = (2.55 cm) 3 = 16.581375 cm3
D = 40.53 g
= 2.44430875
16.581375 cm3
Because 2.55 has 3 sig figs
the final answer is 2.44 g/cm3
Volume of a Irregular Solid
• If you have an irregular solid use
displacement, take initial volume of water in a
graduated flask, submerging the sample in the
flask, and take the final volume while the solid
is submerged. Subtract final volume minus
initial volume.
Examples 3
• Find the density of a solid sample with a mass
of 75.45 g and displacement is used to find
the volume, the initial volume is 20.39 mL, the
final volume is 73.57 mL.
V = 73.57 mL – 20.39 mL = 53.18mL
75.45 g = 1.418766454 g/mL
53.18mL
Since both mass and volume have 4 sig figs
the final answer is 1.419 g/mL
Practice Problems
1. Find the density of a liquid with a volume of
3.8 mL and a mass of 3.75 g.
2. Find the density of a rectangular solid with a
length of 4.5 cm, a width of 2.5 cm, a height
of 1.5 cm and a mass of 47.12 g.
3. Find the density of an irregular solid with a
mass of 1.2 g, an initial volume of 25.7 mL
and final volume of 18.4 mL when
displacement is used.
Answers Practice Problems
1. Density = 0.99 g/mL
2. Density = 2.79 g/cm3
3. Density = 8.8 g/mL