Title:
The molar volume of a gas & R.
(Lab Guidelines: 1,2,3,4,5&6 [attach handout],7,9,10,11,12)
Purposes:
-To use experimental data and the ideal gas law to calculate the ideal gas constant, R.
-To use experimental data and the combined gas law to calculate the molar volume of a gas at
standard temperature and pressure (STP).
-To do even more calculations thought up by your sadistic instructor.
Background material/info:
-What is STP?
-What is Dalton’s law of partial pressures?
-How do I collect a gas over water?
-How does Dalton’s law apply to collecting a gas over water?
Materials:
3-8 cm of Mg ribbon (your instructor will tell you exactly how much to use!), 6.0M HCl, 1000mL extra large graduated
cylinder, thermometer, ruler, one holed stopper, gas collecting tube, copper wire, table of values for the vapor pressure of
water
Procedure:
Lab partner #1:
1. Record the mass/meter of the magnesium ribbon.
2. Cut designated length of magnesium ribbon. Record the length of the magnesium to the nearest 0.01 cm.
3. Attach the magnesium to the copper on the stopper by wrapping it around several times.
Lab partner #2:
1. Pour about 20 mL of HCl into the tube.
2. Fill the remainder of the tube with water up to the very top.
Together
1. Fill the large graduated cylinder or flask about 2/3 – 3/4 full of water.
2. Carefully and firmly place the stopper into the gas collecting tube. Make sure there are no air bubbles in your tube.
3. Invert the tube and quickly place it into the large graduated cylinder or flask. Observe as the HCl makes its way down
the tube and into contact with the magnesium.
4. After the reaction is complete, lift the gas collecting tube so that the water levels in the tube and the large graduated
cylinder/flask are even.
5. Now carefully read the volume to the nearest 0.1 mL and record it in your data.
6. Use a thermometer to record the temperature of the water in the cylinder/flask.
7. Use the table of vapor pressures at the front of the room to record the vapor pressure of water at the temperature you
just recorded.
8. Record today’s barometric pressure of the atmosphere. This will be given to you by your instructor.
Data:
Class discussion will help you figure out what to record.
(Turn over to keep the gases flowing)
Calculations:
Do the necessary calculations to accomplish the 2 stated purposes. Label your calculations. Use appropriate units. Circle
answers. Please make sure this section is organized!!
Questions and More Calculations (including some stoichiometry – YEA!!):
1) Explain how Dalton’s law of partial pressures relates to the collection of a gas over water.
2) Write the balanced equation and net ionic equation for the reaction which took place in the lab today.
3) Assuming you used exactly 20.0 mL of 6.00 M HCl, determine how many mL of HCl remain unused after all the
magnesium has reacted.
4) Using your original mass of magnesium, calculate the number of mL of hydrogen gas which should have formed at
classroom conditions. You will need to use your knowledge of stoichiometry and the ideal gas law in order to complete
this calculation.
5) Now, using your original mass of magnesium, calculate the number of mL of hydrogen gas which should have formed
at classroom conditions. This time you will need to use your knowledge of stoichiometry and the combined gas law in
order to complete this calculation.
NOTE: Your answers to questions 4 and 5 should be identical.
Error Analysis:
Given the accepted values for the molar volume of a gas at STP, and for R, calculate 2 separate percent errors. Also,
discuss possible errors which might have occurred, and their effect on your results.
Conclusion:
Write a conclusion which demonstrates that you understand what you did. How did you accomplish the purposes?
Discuss the math as well.
Remember, here is a great guide for writing your conclusion:
1) Restate the purpose
2) Summarize the procedure and data collection
3) Discuss any important principles / equations used
4) Explain the graphs and/or math
5) Report your results
Also, please include any other interesting information you would like to add.