Charles Law Lab

Quantitative – 50 points

 

Purpose:  To find absolute zero.  The temperature at which all molecular motion stops.

 

Pre Lab Questions:

1.  Temperature is a measure of the average ___________ of molecules.

 

2.  A very massive gas molecule traveling at the same velocity as a lighter gas molecule will have a greater or lesser kinetic energy?

 

3.  What is the mathematical formula for kinetic energy (abbreviation KE)? (look on p. 304)

 

4.  Explain the difference between the velocity of a gas particle and its kinetic energy?

 

5. Air molecules (N2 gas) move at approximately 1150 miles per hour at room temp (25oC).  How come we are not annihilated by these flying air molecules banging into us?

 

6.  At room temperature, are ALL air molecules moving at 1150 MPH?  Explain.

 

7.  Charles’ law says:  If pressure remains constant,  as temperature decreases, volume ________

 

8.  Assume you have collected 50 mL of air over water in a container.  The water’s temperature is 25oC.  At this temperature, water has a vapor pressure of 3.2 kPa.  The TOTAL pressure of the gas in the container is 760 mm Hg (Patmospheric). 

            a.  What is the pressure of just the dry gas in kPa? (Pdry gas)

            b.  Use Boyle’s Law to solve for a corrected volume of the dry gas:  (solve for Vcorrected)

Pdry gasVuncorrected = PatmosphericVcorrected.

 

9.  Why can you not just assume that all 50 mL of air in the container (from problem #8) are just air molecules? (look at the table of water vapor pressures on p. 899 of your textbook if you need help)

 

10.  Assume you have a SEALED 250 mL glass container which contains 0.01 moles of air molecules at room temperature.  You begin to heat it up over a Bunsen burner.  After a few minutes, you decide to count the number of moles of air in the container.  Would you expect to find: 

                 a.  less than 0.01 moles,  b.  more than 0.01 moles,  c. 0.01 moles.  WHY? 

Would you expect the pressure inside the container to be:  

a. greater than outside air pressure,  b.  less than outside air pressure  c.  equal to outside air pressure.  WHY?  

 

11.  Assume the same thing you did in #10, except this time, the container has a hole in the top of it.  After you heated it up, Would you expect to find: 

a.  less than 0.01 moles,  b.  more than 0.01 moles,  c. 0.01 moles.  WHY?

Would you expect the pressure inside the container to be:  

a. greater than outside air pressure,  b.  less than outside air pressure  c.  equal to outside air pressure.  WHY?

 

What might be on the Pre-Lab Quiz?

How hot should the water be in the 600 mL flask?  How do you stabilize the water temp?  Should the Erlenmeyer flask be wet inside or completely dry? 

 

Lab Table Set-up will look like:

 

 

Procedure:

1.  Fill your 600 mL beaker with about 250 mL of water  - not necessary to be EXACT.

2.  Heat the water to a temperature between 70oC and 90oC.  Let the water stabilize (turn off the heat when you reach between 70-90oC.  This will let it stabilize)

3.  Place your EMPTY (NO WATER IN IT!!!  AND IT SHOULD BE VERY DRY INSIDE!!!!!!) Erlenmeyer flask into the 600 mL beaker so that it touches the bottom (but do not block the tube coming out of the rubber stopper).  You will have to hold the Erlenmeyer flask in the water.  Hold it down by putting your fingers on the rubber stopper.

4.  Allow the Erlenmeyer flask to heat up for about 2 minutes.

5.  Record the temperature of the water at the end of these 2 minutes as T1.

6.  Place your finger over the tube and remove the Erlenmeyer flask from the beaker of hot water

    

7.  Turn the Erlenmeyer flask upside down in the bucket of water provided at your table.  When the neck of the flask is in the water, remove your finger from the tube.  (See drawing A below)

Watch the water rush in!  Whoopee!!!!!!

 

 

 

8.  When the water stops rushing in equalize the pressure (see drawing B above).  Make sure you equalize the pressure by allowing the water level in the Erlenmeyer flask to equal that of the water in the tank.

 

9.  Once the pressure has been equalized, put your finger over the hole in the glass tube and remove the flask from the water.  Take off the rubber stopper and take the temperature of the water in the Erlenmeyer flask.  Record this as T2.

 

10.  Pour that water out of the Erlenmeyer flask and into a graduated cylinder to record its volume.  Record  this as Vwater. You can then throw that water away.  You don’t need it.

 

11.  Now, fill the empty Erlenmeyer flask up to the top with water.  Place the rubber stopper into the

Erlenmeyer so that the water fills the glass tube in the rubber stopper.  Pour this water out into a graduated cylinder to record its volume.  Record this as V1.

 

12.  Determine the V2 of gas (V2 is the amount of gas after the temperature is decreased from T1 to T2), use the following equation:  V2 = V1  -   Vwater.  Record as V2uncorrected on your Data Table.

 

 

 

PLEASE READ THIS BEFORE RECORDING ANY DATA:  THE DATA TABLE NEEDS TO BE DONE ON A SEPARATE SHEET OF PAPER.  NEATNESS COUNTS — REMEMBER YOU ARE GOING TO DO THIS EXPERIMENT AT LEAST THREE TIMES, THAT MEANS YOU WILL HAVE THREE DATA TABLES AND THREE LINES ON YOUR GRAPH.          

Data table:  (the stuff in ()’s is there to help you.  Don’t include it in your write-up)

 

V1   _____________mL

T1  ___________oC     ______________K

T2  __________oC      ______________K                                

Vwater ____________mL

V2uncorrected  _____________mL

Room Pressure ________mm Hg (look on barometer in front of room – multiply by 10 since it is in cm)

Water’s vapor pressure at T2 ______________mm Hg (look on p. 899 of textbook)

Pressure of dry gas alone (Room pressure – Water vapor pressure at T2) ____________mm Hg

V2corrected  ________________mL  (see calculation #1 for how to figure this one out)

V1experiment _______________mL   (see calculation #2 for how to figure this one out)

 

How to calculate V2corrected

 

Calculations:

1. You need to calculate the value of V2corrected.  You may be wondering:  What is V2corrected?  Well, I’ll tell you:  V2corrected is the volume the DRY GAS would occupy at standard room pressure (760mm or 101.325kpa).  The V2 you found by subtracting V1-Vwater is the volume of BOTH the DRY GAS and the WATER VAPOR (see my beautiful sketch above).  You need just V2 of the DRY GAS.  I call this V2corrected      

     How do you do this?  

               STEP 1:  V2uncorrected = V1-Vwater

               STEP 2:  Go to the barometer in the front of the room and find the room pressure in mm of Hg.  Record this as room pressure on your data table (record in both mm of Hg and kPa).  This is Patmospheric.

               STEP 3:  Use the procedure you used in the pre-lab (question #8).  GO BACK AND LOOK AT THAT QUESTION RIGHT NOW.  Do you recall how to find Pdrygas?  (Hint:  It has something to do with the barometric pressure of the room, Subtraction, and the chart on p. 899) 

Pdry gasV2uncorrected = PatmosphericV2corrected.

Record this answer as V2corrected on your data table.)  SHOW YOUR WORK FOR DETERMINING V2corrected.

 

 

2.  Using your values for T1,  V2corrected ,   and T2 (Temperatures in Kelvin!), use Charles’ Law (V2T1=V1T2) to solve for V1experiment.  Do this for each of your three trials.  Record this answer as V1experiment on your data table.  Again, don’t forget your values for temperature must be in KELVINS when you use Charles’ Law.

 

What is V1Experiment?  Glad you asked….   You found V1 by filling the Erlenmeyer flask completely with water and dumping it into a graduated cylinder.  As it turns out, this is not a very accurate way to do this.  Therefore, to be more accurate, we use Charles’ Law and the data we collected to get a better value for V1, I call this better value:  V1experiment.  You should find that the answer you get to this calculation should be pretty darn close to the one you got from filling the flask with water and measuring it in the Erlenmeyer flask.

 

Graph:  

 

  Plot a graph of (T1,V1experiment ) and (T2, V2corrected).  The Temperatures are on the X-axis and the Volumes are on the Y-axis.  You will have three lines on your graph – because you did the experiment three times. 

 

The following will help you plot this graph using a program called Graphical Analysis.

 

Charles’ Law Lab

Using Graphical Analysis

 

1.  Find Graphical Analysis on the desktop and open it.

2.  Double Click on the “x” in the x-column of data.  This is in the upper left hand corner of your screen.  VERY UPPER LEFT CORNER IF YOU PLEASE!

3.  Name it “Temperature”

4.  Go down to the UNITS and put in “oC”.  (Hint:  If you use INSERT SYMBOL, you will find a “o”

5.  Click OK

 

6.  Now click on “y” and title it “Volume” with units of “mL”.

 

7.  Enter your numerical values for T2 in “x” and V2corrected in “y”.  Then enter your numerical values for T1 in “x” and V1experiment in “y”.

 

8.  Go up to ANALYZE and choose LINEAR FIT.  A line should appear through your two points which tells you the slope.

 

9.  Go up to ANALYZE and choose ZOOM GRAPH OUT.  You want to keep zooming out until your see the LINEAR FIT line cross the X-axis.  (A cute trick is to click on the magnifying glass with the negative sign in it on the tool bar along the top.)     Notice that the lines DO NOT cross the x axis at 0.  In fact, that 0 on the graph represents 0 volume ONLY, it will be a NEGATIVE value for temperature (BELOW zero!)

 

10. Once you find the X-Axis intercept, go up to ANALYZE and choose INTERPOLATE.  Now, run your mouse arrow over the x-axis intercept area and record what value that is.  This should be the temperature at which the volume drops to zero.  You will need this value to answer question #6 in the POST LAB QUESTIONS

 

11.  Now go up to DATA and choose NEW DATA SET.  A new column of x and y will appear to the right of your original column of data.  Click on these “x”s and “y”s and label them as you did before.  Put in your data for your second trial here. 

 

12.  Move your mouse arrow over to the word “volume” on the graph.  Double click on it.  Now you will need to select DATA SET 2 and check the box for Volume.  What you should see now is two lines on your graph.  If you can’t see both of them, try ANALYZE and AUTO SCALE GRAPH. 

 

13.  Go to ANALYZE and LINEAR FIT for the second set of data.  Where does it hit the x-axis?  (HINT:  If a box comes up and asks you which lines you want to linear fit, you just click on all of them and then it will linear fit each one).

 

14.  Now do this again with your 3rd set of data.

 

15.  When you are all done, call over Mr. Young for inspection.

 

 

Post-Lab Questions:

1.  Find the difference between V1 and V1experiment for each of your three trials. (eg.  Subtract the two values)

 

2.  Using PV=nRT you should be able to figure out how many moles of air were in V1experiment and V2corrected.  .  You solve for n (which is the number of moles of gas).  (Do a-c for each of your three data tables.  Arrange them nice and neat!  Have a-c for one trial together, then do the next a-c, then the next one.  DON’T mix them all together!)

Here are some values you will need to solve PV = nRT:  R= 0.0821 and P = 1 atm.  Temperature is in Kelvins.  The values you will use for V and for T are given in a-c below.

            a. How many moles of air were in V1experiment at a temperature of T2 (room temp)?

            b. How many moles of air were in V1experiment at a temperature of T1 (hot temp)?

            c.  How many moles of air were in V2corrected at a temperature of T2 (room temp)?

 

3.  What do you notice about the answer to 2b. and 2c?  What is significant about these answers?

 

4.  Why did you have to put the Erlenmeyer flask under the water so that the water levels were equal inside and outside the flask BEFORE you put your finger over the glass tube and pulled out the flask?  What would be the result if you had the water level in the flask higher than the water in the bucket?  What about if it were lower?

 

5.  Why does water come rushing into the flask when you turn it over into the water?  Use drawings to explain your answer.  Remember that the pressure of the room pushes down on the water.

 

6.  According to your graph, what is the temperature (in Celsius and Kelvin) at which molecular motion is going to be zero (in other words, zero volume).

 

7.  What is the difference between V2uncorrected and V2corrected.  In other words, what do you have to correct for?  I want to know why you have to correct the volume.

 

What is due for this lab?

 

1.  Pre-Lab Questions:  Questions and answers to Pre-lab (you can just copy-paste the questions from this lab onto a Word file and then type in your answers).

 

2.  Data tables:  Three data tables (one for each trial).  You could just write down all of the information in a column on the extreme left hand side of your paper and then make three columns titled “trial #1”, “trial #2” and “trial #3”.

 

3.  Calculations:  You need to do calculations #1 and #2 three times (one for each trial).  Please make sure they are separated by trial so I can easily read them and figure them out.

 

4.  Graph:  Plot a graph according to the instructions.  It MUST be on graph paper.  Your axis’ must be labeled and include numerical values along the axis.

 

5.  Questions:  Write the questions and then answer them (again, COPY-PASTE is the way to go!).  Show your work for #2 – it is a very important question so don’t skip it!

 

Overall, neatness counts!  Please do a good job on this lab!