image1.wmf image2.wmf Determination of the Molar Mass of a Volatile Liquid by the Ideal Gas Equation
Introduction:
1) The ideal gas law allows chemists to determine and derive important features of compounds by formula manipulation.
a. The formula: PV = nRT
i. P = pressure, V = volume, n = moles, R = ideal gas constant, T = temperature
1. R = 0.08206 L ( atm/(K ( mol)
2) At standard temperature and pressure (STP), the volume for 1 mole of an ideal gas is 22.4 L.
a. TSTP = 273.15K (0.00oC)
b. PSTP = 1 atm (760. mm Hg)
3) Volatile liquids are compounds that can be easily be vaporized.
a. If this vaporization takes place in a known volume and at a given temperature and pressure, the number of moles can be determined.
i. The rearranged formula: n = PV/RT
ii. The pressure in the container is equal to the barometric pressure as long as the container is open to the atmosphere.
1. Remember: 1 atm = 760. mm Hg
b. The molar mass of the volatile liquid can then be determined by knowing the mass of the vapor.
i. Molar mass (M) = mass of vapor/moles of vapor = g/mol.
The Known Volatile Liquids:
1) Acetone (2–Propanone)
2) Cyclohexane
3) Ethanol (Ethyl Alcohol)
4) Ethyl Acetate
5) Isopropyl Acetate
6) Isopropyl Alcohol (2–Propanol)
7) Pentane
Substances to include in the Table of Chemical and Physical Properties:
The boiling point for the seven known volatile liquids listed above.
Procedure:
1) Clean and completely dry a 125 mL Erlenmeyer flask.
2) Obtain a cap for the flask.
a. The cap is an appropriately sized stopper with a hole in the center. A small open glass tube (such as an eye dropper) has been placed through the stopper.
3) Record the mass of the clean and completely dried flask with the cap on an analytical balance.
4) Add approximately 7.5 mL of your volatile liquid to the inside of the flask.
a. Record the name of the volatile liquid assigned.
5) Set up a water bath in a 600 – 1000 mL beaker using a hot plate as a heat source.
a. Place a few boiling stones into the water to promote even heating.
6) Immerse your flask into the water to cover the entire flask without allowing any water to enter the flask.
a. Secure the flask by using a utility clamp.
b. It is very important that water does not get into the flask!
7) Quickly bring the water to a boil and then lower the heat to allow for a gentile boil.
a. Record the temperature of the water once stabilized. (This temperature will also be the heat of your vapor.)
b. Record the pressure in the lab using a barometer.
8) Allow your sample to heat for 10 minutes once a gentle boil has been obtained. Ensure that all liquid material has been vaporized.
9) Remove flask from water bath and allow to cool to room temperature.
a. The vapor may condense back into a liquid; do not be alarmed.
10) Remove ALL TRACES of water from the outside of the flask.
a. Water can condense on the edges of the cap; ensure removal of this water.
11) Obtain the mass of the cool, dried flask on an analytical balance.
12) Dispose of extra/remaining volume of volatile liquid in the appropriately labeled waste containers.
13) Remove cap and ensure the Erlenmeyer flask has been rinsed well with water.
14) Fill up the flask to the top with deionized water.
a. CAREFULLY, transfer the volume into graduated cylinder(s) and record the total volume of the water that the flask could hold.
b. Repeat this volume determination for maximum precision.
15) Repeat experiment for a second trial.
a. Use a separate cleaned and dried 125 mL Erlenmeyer flask.
b. For this trial, add approximately 5.0 mL of the volatile liquid to the inside of the flask.
16) Using the example data table attached, determine the molar mass of your volatile liquid and compare to true value.
Clean – Up:
1) All water baths can be poured down the sink and the boiling stones discarded into the trash.
2) Extra volumes of volatile liquids should be placed in the appropriately labeled waste containers.
Sample Data Table:
a) Volatile liquid assigned: ________________
b) Mass of flask with cap in grams: ________
Trial 1: ________ Trial 2: ________ Trial 3: ________
c) Volume of unknown liquid added (mL):
Trial 1: ________ Trial 2: ________ Trial 3: ________
d) Stable temperature of water (oC):
Trial 1: ________ Trial 2: ________ Trial 3: ________
e) Stable temperature of water (K):
Trial 1: ________ Trial 2: ________ Trial 3: ________
f) Barometric pressure (mm Hg):
Trial 1: ________ Trial 2: ________ Trial 3: ________
g) Barometric pressure (atm):
Trial 1: ________ Trial 2: ________ Trial 3: ________
h) Mass of flask with cap with sample after heating in grams:
Trial 1: ________ Trial 2: ________ Trial 3: ________
i) Mass of volatile liquid in grams (h – b):
Trial 1: ________ Trial 2: ________ Trial 3: ________
j) Volume of water taken up by flask in liters:
Trial 1: ________ Trial 2: ________ Trial 3: ________
k) Moles of volatile liquid using n = PV/RT (mol):
Trial 1: ________ Trial 2: ________ Trial 3: ________
l) Experimental molar mass of volatile liquid in grams per mole (i/k):
Trial 1: ________ Trial 2: ________ Trial 3: ________
m) Average experimental molar mass of volatile liquid: _______________
n) True molar mass of volatile liquid: _______________
o) Percent error: _______________
Model Apparatus Set – Up:
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Barnett & Jones
General Chemistry-I Lab
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Water Fill Line
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General Chemistry – I Lab Barnett
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