Date ______________
Hr __________
Name _____________________________
Molar Volume of a Gas Determined via a Reaction between Magnesium and
Hydrochloric Acid
INTRODUCTION:
Today you will be mixing magnesium metal (Mg) and hydrochloric acid solution (HCl). The reaction
will generate hydrogen gas (H2(g)). The reaction can be written as (you should balance it!):
Mg(s) + HCl(aq)  MgCl2(aq) + H2(g)
This lab will investigate the relationship between the volume of hydrogen gas generated compared to
the moles of hydrogen gas generated – in other words you will be finding the liters per mole of
hydrogen gas. For all trials, the hydrochloric acid will be in excess (assuming you follow directions!).
MATERIALS:
Magnesium metal
2-hole Rubber stopper
Electronic Balance
50mL beaker
HCl (1M)
Vernier gas pressure sensor
Vernier temperature probe
Plastic syringe
125mL Conical Flask
Vernier interface, cables, and PC
10 mL and 100mL Graduated Cylinders
Plastic tubing
Figure 1
PRE-LAB:
1. Set up the lab apparatus as directed by your teacher. See figure 1.
2. Turn on the computer. Double-click the Logger Pro icon. In the Logger Pro software select, File,
Open, Experiment #5 “Molar Volume” in the Advanced Chemistry w Vernier folder.
3. For each trial you will use 5mL of 1M hydrochloric acid (CAUTION-caustic!). You will conduct 3
trials, each time increasing the amount of Mg. See the data table for the amounts to add – under
“Approximate Mass of Mg”.
4. Determine the volume of the flask when it has the stopper in place (hint: figure out how much
water will fit in it!). Think about where this should go on your data table…
5. Obtain and wear goggles
PROCEDURE:
1. Measure the 5mL of hydrochloric acid into the syringe. Do so by placing a small amount of HCl in
a 50-mL beaker and then drawing it up into the syringe. Connect the syringe to the stopper with valve
closed.
2. For trial 1, add 0.1g of Mg to the flask. Record the mass to the nearest 0.001g.
3. Prepare a room temperature water bath in a large beaker. The bath should be deep enough to
completely cover the gas level in the conical flask.
Date ______________
Hr __________
Name _____________________________
4. Connect the temperature probe and gas pressure sensors to the Vernier Lab Pro (green box). Be
sure the cable is connected properly from the Lab Pro to the PC.
5. Connect the gas pressure sensor to the stopper via the plastic tubing and be sure all connections
are snug.
6. In the Logger Pro software make sure that the data-collection rate is set to 1 sample/second and
the length is set at 700 seconds.
7. Submerge the flask in the water bath. Press “Collect” to begin data collection. After about 20
seconds, open the valve on the syringe, press the plunger to add all of the 5mL of hydrochloric acid,
and pull the plunger back to its original position. Close the two-way valve.
8. Keep the flask immersed in the after bath as the reaction proceeds. Once the maximum pressure
is reached you may stop collecting data.
9. Carefully remove the stopper from the flask to relieve the pressure in the flask. DO NOT open the
two way valve to release pressure in the flask.
10. Record the temperature, initial pressure, and maximum pressure in the data table before
continuing.
11. Repeat for trials 2 and 3. Empty and then rinse the flask with deionized water between trials.
12. When finished put away all equipment in the places indicated by your teacher.
Data Table 1
Approximate Mass of Mg (g)
Trial 1
Trial 2
Trial 3
0.01
0.02
0.03
Actual Mass Mg Used (g)
Moles Mg Used
Moles H2 Generated
Volume H2 Generated (L)
Temp. (K)
Initial Pressure (kPa)
Maximum Pressure (kPa)
Pressure change (kPa)
CALCULATIONS:
1. Calculate the moles of Mg used for each trial and enter it in the data table.
2. Using the balanced equation and stoichiometry, calculate the moles of H2 gas generated. This is
a theoretical amount, but for purposes of this lab we’ll assume it’s the same as the actual amount in
your flask.
Show work for calculations #1-2 for one of your trials in the space below:
Date ______________
Hr __________
Name _____________________________
QUESTIONS/ANALYSIS:
Molar Volume of a Gas
Gases under what we call “ideal” conditions behave the same – they will take up the same volume
and exert the same pressure assuming all other things are equal. In essence, it does not matter what
type of gas atom or molecule it is. Carbon dioxide (CO2) will behave the same as Hydrogen (H2),
which will behave the same as Xenon (Xe).
Because gas volumes/pressures change if temperature or pressure changes, chemists use
conditions called Standard Temperature and Pressure (STP) as a way of comparing gases. STP is
273K (0°C) and 1.00 atm of pressure. If we always correct to STP then we will always be comparing
gases under the same conditions.
3. Before you can determine the molar volume of a gas, we must correct the conditions in our lab to
STP. You can use the combined gas law to correct the volume of gas you obtained to STP (P2 and
V2 are STP, solve for V2). Your P1, V1, and T1 values are in your Data Table 1 above. Be sure you
think carefully about which value is which.
PV P V
Combined Gas Law: 1 1 = 2 2
1atm = 101.325kPa
T1
T2
Data Table 2
Trial 1
Trial 2
Trial 3
Volume of H2 gas at STP (L)
Show your work for at least one of your trials here:
4. Now that you have your volumes corrected to STP you can calculate the Molar Volume of a gas
(in other words, the volume of one mole of your gas). To do this, simply divide the volume by the
moles for each trial.
Data table 3
Trial 1
Trial 2
Trial 3
Average
Molar Volume (L/mol)
Show your work for at least one of your trials here:
5. The accepted value for the Molar Volume of a gas at STP is 22.4 L/mol. Compare your average
Molar Volume to the accepted value by calculating a percent error. Show your work.
Date ______________
Hr __________
Name _____________________________
The Ideal Gas Law
The ideal gas law relates pressure, volume, moles, and temperature. Another way we can check our
lab results is by calculating R, the gas constant, from our lab data. Use your lab data to find your
value for the gas constant.
PV = nRT or R = (PV)/(nT)
Data Table 4 (you can find the data for this table in previous data tables)
Trial 1
Trial 2
Trial 3
Temperature (K)
Volume H2 (L)
Pressure of H2
(kPa)
Moles H2
R (L•kPa/mol•K)
Average =
6. The accepted value for R is 8.314 L•kPa/mol•K. Compare your average value for the gas constant
to the accepted value by calculating a percent error. Show your work.