How to Calculate Density - Worked Example Problem

Finding the Ratio Between Mass and Volume

Density column showing colored liquids with different densities.
Density column showing colored liquids with different densities. Dorling Kindersley: Dave King / Getty Images

Density is the measurement of the amount of mass per unit of volume. In order to calculate density, you need to know the mass and volume of the item. The mass is usually the easy part while volume can be tricky. Simple shaped objects are usually given in homework problems such as using a cube, brick or sphere. The formula for density is:

density = mass/volume

This example problem shows the steps needed to calculate the density of an object and a liquid when given the mass and volume.

Question 1: What is the density of a cube of sugar weighing 11.2 grams measuring 2 cm on a side?

Step 1: Find the mass and volume of the sugar cube.

Mass = 11.2 grams
Volume = cube with 2 cm sides.

Volume of a cube = (length of side)3
Volume = (2 cm)3
Volume = 8 cm3

Step 2: Plug your variables into the density formula.

density = mass/volume
density = 11.2 grams/8 cm3
density = 1.4 grams/cm3

Answer 1: The sugar cube has a density of 1.4 grams/cm3.

Question 2: A solution of water and salt contains 25 grams of salt in 250 mL of water. What is the density of the salt water? (Use density of water = 1 g/mL)

Step 1: Find the mass and volume of the salt water.

This time, there are two masses. The mass of the salt and the mass of the water are both needed to find the mass of the salt water. The mass of the salt is given, but the only the volume of water is given. We've also been given the density of water, so we can calculate the mass of the water.

densitywater = masswater/volumewater

solve for masswater,

masswater = densitywater·volumewater
masswater = 1 g/mL · 250 mL
masswater = 250 grams

Now we have enough to find the mass of the salt water.

masstotal = masssalt + masswater
masstotal = 25 g + 250 g
masstotal = 275 g

Volume of the salt water is 250 mL.

Step 2: Plug your values into the density formula.

density = mass/volume
density = 275 g/250 mL
density = 1.1 g/mL

Answer 2: The salt water has a density of 1.1 grams/mL.

Tips for Density Calculations

In some cases, the mass will be given to you. If not, you'll need to obtain it yourself by weighing the object. When obtaining mass, be aware of how accurate and precise the measurement will be. The same goes for measuring volume. Obviously, you'll get a more precise measurement using a graduated cylinder than using a beaker, however, you may not need such a close measurement. The significant figures reported in the density calculation are those of your least precise measurement. So, if your mass is 22 kg, reporting a volume measurement to the nearest microliter is unnecessary.

Another important concept to keep in mind is whether your answer makes sense. If an object seems heavy for its size, it should have a high density value. How high? Keep in mind the density of water is about 1 g/cm³. Objects less dense than this float in water, while those that are more dense sink in water. If an object sinks in water, your density value better be greater than 1!

More Homework Help

Need more examples of help with related problems?

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Helmenstine, Anne Marie, Ph.D. "How to Calculate Density - Worked Example Problem." ThoughtCo, Dec. 6, 2017, thoughtco.com/how-to-calculate-density-609604. Helmenstine, Anne Marie, Ph.D. (2017, December 6). How to Calculate Density - Worked Example Problem. Retrieved from https://www.thoughtco.com/how-to-calculate-density-609604 Helmenstine, Anne Marie, Ph.D. "How to Calculate Density - Worked Example Problem." ThoughtCo. https://www.thoughtco.com/how-to-calculate-density-609604 (accessed December 16, 2017).