Background: Can We Discover a Relationship Between the Pressure of a Gas and Its Volume

LAB 4

BEHAVIOR OF GASES

Background: Can we discover a relationship between the pressure of a gas and its volume? In this experiment a given volume of gas will be placed in a sealed syringe and the pressure on this gas will be increased by increasing the number of books on top of the syringe. This will not be the total pressure exerted on the syringe; there is also air pressure that needs to be accounted for. So,

total pressure = pressure of books + air pressure

(Ptotal) = (Pbooks) + (Pair)

You will record the volume of trapped air in the syringe with each successive number of books added to the syringe. These data will be plotted on two graphs in order to “see” the patterns and determine a simple mathematical relationship between pressure and volume.

This lab will cover lab obj. 1,3,6,7,8,9,10

unit 2 obj. 1,12

PART 1: qualitative observations

Each student does this part individually.

1. Place the plunger at about 35.0 cc and place your finger over the end of the syringe. Push on the plunger as hard as you can and describe what you observe.

2. Now remove your finger from the end of the syringe and hold it near your ear. Push on the plunger now and describe what you observe.

3. With the plunger at the bottom of the syringe place your finger over the hole and attempt to remove the plunger. Describe your observations.

QUESTIONS

PART 1

1. Was the syringe in step one empty? What is your

evidence?

2. When you pushed on the plunger in step 1 what

opposed its movement?

3. What opposed the movement of the plunger in

step 3?

PART 2 quantitative relationship between pressure and volume

You will now work in lab pairs, making sure to switch jobs with each new trial.

1. Place the plunger at 35.0 cc and set up the apparatus as shown below:

books

clamp

syringe

stopper

ring stand

2. Carefully place a book on top of the plunger, making sure it does not rub against the ring stand and you do not support it too much with your hands. (remember to use the same edition of the textbook throughout the lab). Read the corresponding volume as precisely as possible (to the tenths column) and remember to record the uncertainty, use our rule of thumb.

3. Now repeat step 2 with a second book and record the volume. Repeat this process until a total of 7 books have been placed on the plunger.

4. This now completes one trial, repeat the process until a total of 5 trials have been completed. It is important to dismantle the syringe and plunger in between trials and reset the plunger at 35.0 cc.

YOU MAY USE THE DATA TABLE PROVIDED IN YOUR PACKET, REMEMBER TO LEAVE THE SECOND TABLE BLANK UNTIL YOU’RE SURE OF THE DATA AND CALCULATIONS.

Calculations :

With all of the data we now have we need to find a pattern or regularity in it. First fill out the rest of the data table according to the following instructions and then construct 2 graphs using that information.

DATA TABLE COMPLETION

(HINT: watch sig. figs. throughout the table)

1. Find average volume for each book by dividing the total volume of each trial with a certain number of books and dividing by the number of trials. Do this for each amount of books placed on the syringe. Don’t forget to record the uncertainty.

2. Transfer the average volumes to the corresponding column on the lower half of the table.

3. In the columns labeled V+ and V- record the average volume plus the uncertainty and average volume minus the uncertainty.

4. In the columns labeled 1/V, 1/V+ and 1/V- determine the reciprocal of the columns labeled V, V+ and V-.

In order to finish filling in the data table you will need to construct 2 graphs. Use the following instructions.

(HINT: don’t lose points by failing to use proper labels, scales, units and titles on your graphs.

GRAPH: PRESSURE vs VOLUME

1. Construct a full page graph using pressure as the label on the vertical axis and volume on the horizontal axis. Remember to keep the axes in a few spaces and to start the scales at the origin. Instead of placing the uncertainty on the axes you will show it graphically according to your instructor’s directions.

GRAPH: PRESSURE vs RECIPROCAL VOLUME

1. Construct a full page graph using pressure as the label on the vertical axis and the reciprocal of volume (1/V) on the horizontal axis. Again remember to move the axes in a few spaces and also graph the uncertainty as per your instructor’s directions.

IMPORTANT: This time the horizontal axis should be placed about a third of the way up the vertical axis, again this is the origin of the graph. This is important as you will discover later.

Some questions to think about after the second graph is constructed.

1. Is there another data point you know of but did not plot? (There is --- do it now)

2. What is different about this graph compared to the first graph?

3. Why are you more comfortable with the second graph than the first graph?

4. Have you worked with this type of graph before? Do you know its equation?

5. When books were placed on the plunger, why didn’t the plunger go to the bottom of the syringe?

6. Did the trapped air want to escape the syringe?

7. Why didn’t it push the plunger out of the syringe? (Think back to step 3 part 1)

8. If there was truly no pressure on the trapped gas what would be its corresponding volume? (Think of what would happen if the syringe with no books were taken into outer space. Obviously if there was absolutely no pressure on the trapped gas its corresponding volume would be infinite and 1/V would = 0; therefore our graph doesn’t truly show when pressure = zero. You can determine this in the following manner:

A. Extend the straight line on your graph until it intersects the y-axis. At this y-intercept 1/V = 0, therefore the pressure on the trapped gas = 0. But you already have a pressure scale--what must the distance between the “old” origin and the y-intercept be equal to? Right, the missing pressure caused by the air in our room! But pressure can’t be a negative value, nature can’t owe you!

B. To the left of the original pressure scale construct a new scale with the y-intercept = origin. This creates an absolute scale, no negatives.

C. Look at the graph when no books were on the syringe-read across to the new scale-this is equal to pressure due to air in books!

D. Look at the graph when one book was placed on the syringe. Read across to the new pressure scale; you really had more than one book of pressure. This new scale shows the total pressure :

Pt = Pa + Pb

COMPLETING THE DATA TABLE

1. Now complete the data table based on the results from the graph P vs 1/V and the headings in the last 4 columns.

QUESTIONS PART 2

1. What relationship exists between the product of the total pressure x volume? (last column of the data table)