Experiment: 9
College Board Correlation #5
Determining the Molar Volume of a Gas
AP Chemistry
Blaine High School
Your Full Name Here
Full Due Date
Purpose
Write a brief summary of the purpose of the lab. No more than two to three sentences.
Procedure
Write a summary of the lab procedure that you did. This can be written in first person form and
should be written in paragraph form. Be sure to include all pertinent measured values and units.
Data Table
Trial 1
Trial 2
Trial 1
Trial 2
Mass of Mg
Evidence of Chemical Reaction
Volume of H2 Gas
Temperature of Water Bath
Barometric Pressure
Results Table
Theoretical number of moles of H2
gas
Vapor pressure of water
Partial pressure of H2 gas
Calculated volume of H2 gas at STP
Molar Volume of H2 gas
Average molar volume
Post-Lab Calculations and Analysis
Construct a Results Table to summarize the results of the following calculations. Provide a
sample calculation for all values calculated. This can be included with each question or attached
on a separate sheet of paper.
1. Calculate the theoretical number of moles of hydrogen gas produced in Trials 1 and 2.
2. Use Table 1 in the Background section to find the vapor pressure of water at the temperature
of the water bath in this experiment. Calculate the partial pressure of hydrogen gas produced in
Trials 1 and 2.
3. Use the combined gas law to convert the measured volume of hydrogen to the “ideal” volume
the hydrogen gas would occupy at STP for Trials 1 and 2. Hint: Remember the units!
4. Divide the volume of hydrogen gas at STP by the theoretical number of moles of hydrogen to
calculate the molar volume of hydrogen for Trials 1 and 2.
5. What is the average value of the molar volume of hydrogen? Look up the literature value of
the molar volume of a gas at STP in your textbook and calculate the percent error in your
experimental determination of the molar volume of hydrogen.
6. one mole of hydrogen gas has a mass of 2.02 g. Use your value of the molar volume of
hydrogen to calculate the mass of one liter of hydrogen gas at STP. This is the density of
hydrogen in g/L. How does this experimental value of the density compare with the literature
value?
7. In setting up this experiment, a student noticed that a bubble of air leaked into the eudiometer
tube when it was inverted in the water bath. What effect would this have on the measured
volume of hydrogen gas? Would the calculated molar volume of hydrogen be too high or too low
as a result of this error? Explain.
8. A student noticed that the magnesium ribbon appeared to be oxidized—the metal surface was
black and dull rather than silver and shiny. What effect would this error have on the measured
volume of hydrogen gas? Would the calculated molar volume of hydrogen be too high or too low
as a result of this error? Explain.
Conclusions
Make a simple statement concerning what you can conclude from the experiment. Use data to
support your conclusions.
Discussion of Theory
In this section you should include such information as: What theory was demonstrated in this
experiment? What do the calculations show? How was the purpose of the experiment fulfilled?
Why does (or doesn’t) the experiment work? Refer back to the purpose of the lab to write this
section.
Experimental Sources of Error
What are some specific sources of error, and how do they influence the data? Do they make the
values obtained larger or smaller than they should be? Which measurement was the least precise?
Instrumental error and human error exist in all experiments, and should not be mentioned as a
source of error unless they cause a significant fault. Significant digits and mistakes in
calculations are NOT a valid source of error. In writing this section it is sometimes helpful to ask
yourself what you would do differently if you were to repeat the experiment and wanted to
obtain better precision. If you can calculate a percent error or percent deviation, do so and
include it in this section.