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Exercise 9:
Determining a Molar Mass
Introduction
In this experiment you will determine the molar mass of a sample of a volatile liquid
using the ideal gas law.
PV = nRT
The ideal gas law can be rearranged to look like the following:
n
PV
RT
where:
n = number of moles (moles)
P = pressure of the gas (atm)
V = the volume the gas occupies (L)
R = the universal gas constant
T = the temperature of the gas (K)
In this experiment, you will measure P, V, and T of a known mass of a gas. Knowing the value
of R, you will calculate the number of moles of the gas, n. As you recall from lecture, the
number of moles of a substance can also be derived from:
n
mass of substance
molar mass
Since you know the mass of the gas used, and the number of moles of the gas, you can rearrange
the above equation to calculate the molar mass of the gas.
Procedures
1. Place wire gauze on an iron ring on a ring stand. Half fill a 600 mL beaker with water,
add several boiling chips, and place the beaker on the wire gauze. Attach a second iron
ring to the ring stand and place the beaker in the second iron ring (this is to prevent the
beaker from toppling over). Begin strongly heating the water with a bunsen burner.
2. Obtain a clean, dry 125 mL Erlenmeyer flask. Place a piece of aluminum foil over the
top of the flask. Use a rubber band to hold the foil in place. Using a pin, poke several
holes into the aluminum foil lid.
3. Determine the mass of the flask, foil and rubber band to the nearest 0.01 g. Record the
mass, with units in the Data Table (1).
4. Obtain an unknown liquid from your instructor. Record the unknown letter in the Data
Table.
5. Remove the rubber band and aluminum foil and place about 3 mL of your liquid into the
flask. Place the aluminum foil lid back on the flask and secure it with a rubber band.
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6. Attach a utility clamp to the neck of the Erlenmeyer flask. Lower the flask into the
boiling water. Lower the flask enough to keep the level of the water around the flask as
high as possible, but do not allow any of the water to enter into the flask. Make sure that
neither the flask nor the clamp touches the beaker wall. Secure the clamp to the ring
stand
7. Lower the heat to just maintain boiling in the boiling water bath
8. When the liquid in the flask can no longer be seen, continue heating for an additional 5
minutes.
9. Determine the temperature of the water in the beaker to the nearest 0.1 ºC. Record the
temperature, with units in the Data Table (4 & 5). The temperature of the gas in the flask
will be the same as the temperature of the water surrounding it.
10. Remove the flask, wipe it dry, and allow it to cool to room temperature. A drop of liquid
may condense inside the flask. Dry any water on the Erlenmeyer flask and determine the
mass of the flask, foil, rubber band and liquid to the nearest 0.01 g. Record the mass in
the Data Table (2). You now have enough information to determine the mass of the gas
sample (3).
11. Since the flask was open to the air, the pressure of the gas in the flaks is equal to
atmospheric pressure. Using a barometer, measure the atmospheric pressure. Record the
atmospheric pressure in the Data Table (6 & 7).
12. Fill the flask to the brim with water. Using your 100 mL graduated cylinder measure the
volume of the Erlenmeyer flask. Record the volume of the graduated cylinder in your
Data Table (8 &9).
13. Repeat the experiment a second time (steps 4 through 11). If time permits repeat the
experiment a third time.
14. You know have enough information to calculate n (the number of moles of gas). Using
your experimental values, and the known value of R, calculate n for your gas (10) (be
careful with units).
15. Since you know the mass of sample, and the number of moles of sample, you can
calculate the molar mass of the sample (11).
16. Calculate the average molar mass that you obtained from your sample.
17. Obtain the accepted value of the molar mass of your sample from your instructor and
calculate your percent error.
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Data Sheet for Exercise 9:
Unknown Liquid Letter:
Determining a Molar Mass
_________
Trial 1
Trial 2
Trial 3
1. Mass of Flask + foil + rubber band (g)...……….
_________
_________
_________
2. Mass of Flask + foil + rubber band + vapor (g)
_________
_________
_________
3. Mass of the vapor (g)……………..……………..
_________
_________
_________
4. Temperature of the boiling water (ºC)…………
_________
_________
_________
5. Temperature of the boiling water (K).…………
_________
_________
_________
6. Barometric Pressure (torr)……………………...
_________
_________
_________
7. Barometric Pressure (atm)…...…………………
_________
_________
_________
8. Volume of flask (mL)……………………………
_________
_________
_________
9. Volume of flask (L)……………………………...
_________
_________
_________
10. Moles of vapor, n (from PV = nRT) (mol)........
_________
_________
_________
11. Molar mass of sample (g/mol)…………………
_________
_________
_________
Average Molar mass of sample:
_________
Accepted value of the molar mass:
_________
Percent Error of the Molar mass:
_________
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Exercise 9:
Determining a Molar Mass, Pre-Lab Questions
1. Fill in the blanks in the following tables and determine the average molar mass of the sample.
Trial 1
Trial 2
Trial 3
1. Mass of Flask + foil + rubber band (g)...……….
__53.950__
__53.950__
__53.950__
2. Mass of Flask + foil + rubber band + vapor (g)
__54.034__
__54.027__
__54.021__
3. Mass of the vapor (g)……………..……………..
_________
_________
_________
4. Temperature of the boiling water (ºC)…………
___99.4___
___99.6___
___99.8___
5. Temperature of the boiling water (K).…………
_________
_________
_________
6. Barometric Pressure (torr)……………………...
___754____ ____755___ ___755____
7. Barometric Pressure (atm)…...…………………
_________
_________
_________
8. Volume of flask (mL)……………………………
___124.5__
___124.2__
___124.9__
9. Volume of flask (L)……………………………...
_________
_________
_________
10. Moles of vapor, n (from PV = nRT) (mol)........
_________
_________
_________
11. Molar mass of sample (g/mol)…………………
_________
_________
_________
12. Average Molar mass of sample: _________
13. If the true molar mass of the sample was 18.02 g/mol, what is the percent error of the molar
mass?