1-3 Activity: Finding Density

Purpose

What is the density of granite, a type of rock commonly found in the Canadian Shield?

To find density, first look at the original formula for it: D = m/V

This activity will use several samples of granite rocks of different sizes. Looking at the formula above, we need to find the mass (m) and the volume (V) of each of these rocks. Before we look at the data, you need to know how it's gathered.

 Finding Mass

Finding mass is relatively simple: put it on a scale of some kind. Quite often we use an electronic balance, which can give a reading to the nearest gram, tenth of a gram, or even more precise than that.

This scale says 128.7 g, which is the mass we'll use in the example. If you look at this result, it also means that we know the mass down to the nearest 0.1 g.

Finding Volume

This is going to be a little more tricky, and it uses something called Archimedes' Principle. Watch how the red water's level changes, from about the 27-second to 40-second mark of this video. The demonstrator puts a mass into the tall container of red water, and when he takes it back out, the water level drops. When he puts it back in, the water level rises.

We can summarize this part of the Principle like this:

Applying this idea to our experiment, we put some water into a graduated cylinder: a tall, skinny container which volume markings on the side. Below is a variety of sizes you might see in a lab.

So let's use the example below to practice finding the volume of a strangely-shaped object. You can estimate in-between the marks you see to get it down to the nearest millilitre (mL).

Original volume V1  = 45 mL

Final volume V2 = 78 mL

(Not everyone will estimate these exactly the same, so if you're off by one or two millilitres, that's fine.)

Volume of object: V = V2 – V1 = 78 mL – 45 mL = 33 mL

Since the object we're looking at is a solid, you don't use millilitres (mL) for the unit of volume. Instead, you use cubic centimetres (cm3)... but it's easy to convert, because 1 cm3 = 1 mL. This means the object's volume is 33 cm³.

Calculating Density

Now that you have mass and volume, you can calculate density:

D = m/V = (128.7 g)/(33 cm³) = 3.9 g/cm³

To review, these are the steps:

1. Measure the mass of the object.

2. Measure the volume in the graduated cylinder before and after adding the object.

3. Subtract these volumes to find the volume of the object.

4. Calculate density using the formula.

Raw Data

Here are the masses and volumes measured by the experimenter.

Calculated Data

Copy the chart below onto paper. Put the masses in from the chart above, and calculate the volume for each step.

Once you have all the masses and volumes, calculate the density for each sample. Then, find the average density by adding all the individual densities up, and dividing that sum by the number of samples.

Conclusion

Look back at the Purpose section, and use your results to answer the question:

The average density of granite is ________________ .