Title:
(Lab Guidelines: 1,2,3,4,5&6[attach handout],7,9,10,11,12)
The molar volume of a gas & R.
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 a barometer? How does it work?
-Deriving the Ideal Gas Law
-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:
Length of Mg ribbon as directed by your instructor, 6.0M HCl, large oversized graduated cylinder or flask, thermometer,
ruler, one holed stopper, gas collecting tube, copper wire
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 and the procedure will help you figure out what to record.
Calculations & Questions & Error Analysis:
1. Calculate the mass of your magnesium ribbon using the length of ribbon used and the mass per meter given in the data
table.
2. Calculate the number of moles of magnesium which you used in the reaction.
3. Since the hydrogen gas was collected over water, two gases were actually present: hydrogen and water vapor.
Calculate the partial pressure of the hydrogen gas collected by subtracting the vapor pressure of the water from the
barometer reading pressure.
4. Write the balanced equation for the reaction which took place in this lab
5. Use the equation from #4 and the moles of magnesium reacted to determine the number of moles of hydrogen gas
produced in this lab.
6. Use the Ideal Gas Law (PV=nRT) and your data to determine the value of the Ideal Gas Constant, R (include units).
Remember: when using PV=nRT, pressure must be in atm, volume must be in liters and temperature must be in Kelvin.
Also, remember that you must use the partial pressure of the hydrogen gas which you calculated in #3.
7. Calculate your % error for R. (theoretical value is 0.0821 L atm / mol K)
8. Using the combined gas law, calculate the volume of hydrogen gas you would have collected if this experiment had
been performed so that the hydrogen gas was at STP.
9. Calculate the molar volume of hydrogen gas at STP in units of L/mol.
10. Calculate your % error for the volume calculated in #9. (theoretical value is 22.41 L/mole)
11. One final mean question. You added 20 mL of 6.00 M HCl to the magnesium. How many mL of the 6.00 M HCl
actually reacted? How much HCl was left over unreacted? (Hint: You need to use the new stoichiometry road map
presented with the penny lab.) So, is HCl the excess or the limiting reactant? Explain.
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.