Name ___________________________________ period _____
Lab: Molar Mass of a Volatile Liquid (16 points)
(page 1 of 2)
Objective: determine the molar mass of a volatile liquid
Reference: Zumdahl Chapter 5 section 4
Introduction: If a liquid is volatile (evaporates easily), its molar mass can be determined by
heating the sample until it vaporizes and assume it behaves as an ideal gas. The following can
be shown: molar mass = dRT/P or molar mass = mRT/VP Where d is gas density, R is the
ideal gas constant, T is gas temperature in Kelvin, P is gas pressure, and V is gas volume.
In this study the liquid sample will be placed in a test tube and sealed with a 1-holed stopper.
The sample is placed in a boiling water bath. The sample vaporizes, vapors fill the test tube
and excess escapes through the small hole. It is reasonable to assume the vapor temperature is
the same as the boiling water. Since the test tube is open to the air it is reasonable to assume
the pressure of the vapor will be the same as atmospheric pressure. The vapor volume is the
same as the test tube and it can be easily measured. After boiling, the vapor is quickly
condensed with an ice bath to prevent it from escaping the test tube. The condensate mass
and vapor mass should be the same. Use a very sensitive balance to determine the mass of the
condensate.
Hazards: all unknown are flammable; several are toxic
Equipment: test tube 18x150mm, 1-hole stopper, hot plate, tall boiling water bath, tall ice
bath, fume hood, analytical balance
Procedure:
1) Record mass of clean test tube and 1-hole stopper
2) See instructor for about 40 drops on unknown
3) In the fume hood, clamp sample in a boiling water bath. The test tube should be
immersed as far as possible into the boiling water. Expanding vapor should push the
air from the test tube. Excess vapor should escape into hood.
4) Record temperature and pressure of vapor sample.
5) Once all the sample has vaporized quickly cool in ice bath ~5 minutes.
6) Record the mass of condensed sample.
7) Calculate volume of vapor sample. Clean test tube and stopper. Record the mass of
this test tube and stopper that is filled w/ water. The density of water is 1.00g/mL.
Record the vapor sample volume.
Data Table: (2 points)
Unknown letter
Show smooth calculation for molar mass
unknown (6 points):
Mass test tube & stopper
Temperature vapor sample
Pressure vapor sample
Mass test tube, stopper, condensate
Mass condensate (or vapor sample)
Mass test tube, stopper, water
Volume vapor sample
Questions: (2 points each)
1) Explain why the ideal gas law can be used to determine the molar mass of the liquid?
2) Vapors are not ideal. What measurement will be most affected by this? Will this
cause the calculated molar mass calculation to be too high or too low?
3) Did all the vapor condense into a liquid? Again, if not, how did this affect the
calculated molar mass?
4)
It was not necessary to be precise when the liquid was measured out into the test tube.
Why?
Name ___________________________________ period _____
Pre Lab Homework: Molar Mass of a Volatile Liquid (2 points)
(Page 2 of 2)
1) The following data was obtained:
Volume of gas: 8.352mL
Pressure of atmosphere: 742mmHg
Temperature of boiling water
Mass condensate: 0.0326g
Show a smooth calculation for the molar mass of the sample.
2) The following mistakes were made when carrying out this lab. What effect does each
have on the calculated moral mass? Be specific. For example, too large because…,
too small because…, or no effect because….
a) Only half of the test tube was immerged in the boiling water bath, so the upper half
of the sample was less than 100’C.
b) Four mL of the unknown was placed in the test tube instead of the recommended
2mL.
c) The mass of the condensate was not determined quickly. Instead, the test tube was
allowed to stand around for a while before it was massed.