Sci-ber Text - 7th Grade

Matter - Mass the Gas

It is usually easy to find the mass and volume of different solids and liquids. To find the mass of a solid, just place it on the scale.

Finding the mass of a liquid is a little more tricky. First, find the mass of an empty container that will hold the liquid. Then, fill the container with the designated amount of liquid, and weigh it on the scale. The last step is to subtract the mass of the empty container from the mass of the container with the liquid. The difference is the mass of the liquid!!!

Finding volume requires a different set of techniques. Pour the volume that you want to find in a graduated cylinder or a beaker and read the volume in milliliters (mL).

Measuring the volume of solids is a different process. If the solid in question is a regular square/rectangular shape, then you can find the volume by measuring it's length, width, and height. Multiply those three measurements, and you have found the volume. Make sure that your measurements are in centimeters (cm). This way your volume measurements are AUTOMATICALLY in volume units. In other words, one cubic centimeter (cm³) is the SAME as one milliliter (mL), which is the unit of measure displayed on graduated cylinders and beakers. If the solid is an irregular shape, the volume displacement method needs to be used. Use a beaker or graduated cylinder that can hold the solid. Put a known amount of water into the container, and record this volume. Next, place the solid to be measured in the container, measure and record the volume of the water/solid combination. Subtract the original volume from the combined volume, and the difference is the volume of the solid.

But how do you measure the volume of a gas? How do you measure the mass of a gas? Does a gas like air, have enough mass to measure in the lab? Does air weigh anything? It must weigh something, since it IS matter, and matter has both volume AND mass.

This activity will give you a way to estimate the mass and volume of a gas like air. What kind of container can we use to hold the air? A beaker? A graduated cylinder? It would just leak out, and they are probably already filled with air, so these containers wouldn't work too well. We will use a balloon, since it is made just for this purpose!

Problems:
How can you estimate the mass of air? How can you estimate the density of a quantity of air? Will the density measurements change as the amount of air in the balloon increases.

Materials (per group):

1-2 balloons
Balance or scale with 0.1 g accuracy
Tape (to keep the balloon on the scale)
String
Metric ruler or meter stick
Calculator
Pencil or pen, paper, and graph paper

Procedure:

Obtain the materials from your teacher.
Take a small piece of tape, make a loop with the sticky side out, and place it on the scale tray. Zero your scale or balance with the tape on the tray so that the tape will not be massed.
Place a deflated balloon on the scale and find its mass. Record this mass on your data table.
Roll the deflated balloon up into a ball, and measure its circumference, or the distance around the widest part. To do this, take a piece of string and wrap it around the balloon at the widest point from side to side. Mark the string where it meets the end, or just hold it at the point where it met the end. The length of the string you are holding is the circumference of the balloon. Measure this length with the ruler or meter stick and record it on your data table.
To find the volume of this balloon "ball," we will use Euclid's formula for the volume of a sphere, and a formula to relate the circumference that we have measured to the radius. There is some math involved in finding a simple version of the formula to use. If you would like to see how we came to our simple version, click here. The simplified formula for the volume of a sphere is:

V = C*C*C/59.2

where C is the circumference that you measured. Use a calculator to determine the volume (V) in cubic centimeters (cm³ or mL) of your balloon.
Record this volume in the data table.
Take the balloon and inflate it with one lung full of air.
Find the mass of the inflated balloon by zeroing the scale and placing the balloon on the tape so that it does not roll off. Record this mass on the data table.
Find the diameter of the balloon the same way as you did before, without rolling it up into a ball first. Again, you need to measure the widest point around the balloon with the string. Measure the length of string and record this circumference in the data table.
Repeat the steps for finding the mass and volume of the balloon with two and three lungs worth of air.
Repeat the measurements and calculations, recording the data in the table as you go.
Calculate the density of each set of measurements. The formula for density is: D=m/v, or the density(D) is equal to the mass(m) divided by the volume(v). Take the mass of each measurement and divide it by the volume for that measurement. This will give you the density of the balloon and air in g/cm³. Record these densities in the data table.

Safety concerns: Be sure to follow all chemical, heat, and glassware safety rules that are specified by your teacher in all general laboratory experiences.

Data:

Balloon Breaths	mass (g)	circumference (cm)	volume (cm³ or mL)	density (g/cm³ or mL)
0	__________ g	____________ cm	____________ cm³	____________ g/cm³
1	__________ g	____________ cm	____________ cm³	____________ g/cm³
2	__________ g	____________ cm	____________ cm³	____________ g/cm³
3	__________ g	____________ cm	____________ cm³	____________ g/cm³

Analysis:

Did the weight of the balloon change as you put more and more air in it?
How much did each of your breaths weigh (one breath vs. two breaths vs. three breaths)?
Make a graph of two of the following:
- number of breaths vs. mass
- number of breaths vs. circumference
- number of breaths vs. volume
- number of breaths vs. density
What shape is the line that each graph makes? Are they straight lines?
The density of air is 0.0012 g/cm³. How close do your measurements come to this figure?
Does the mass of the balloon affect your density measurements? Could you change your measurements to adjust for this?
Does the balloon compress the air inside? If so, would that make the density of the air increase or decrease?