CHEMISTRY 161 – EXPERIMENT 8
MOLAR MASS DETERMINATION OF GASES & SOLUTIONS:
Flick Your Bic & Freezing Point Depression
INTRODUCTION:
PART I: Flick your Bic
In this experiment we are going to determine the molar massof the gas in a Bic lighter. To accomplish this
we will use the Ideal Gas Law which accurately predicts the behavior of most gases for pressures less than a few
atmospheres and temperatures well above the boiling point of the gas. The Ideal Gas Equation is defined as
follows:
PV  nRT
where:
P = pressure of a gas in atmospheres
V = volume of a gas in Liters
n = moles of a gas present
T = Temperature of a gas in Kelvin
R = gas constant, 0.0821 L atm / mole K
Another way of expressing the number of moles of gas present is to divide the grams of gas by the molar
massof the gas.
n
grams of gas
g

molecular weight of gas
MW
Upon substitution of this value into the Ideal Gas Equation and algebraic rearrangement, the following equation
results.
MW 
gRT
PV
We are now in a position to experimentally determine the molar massof the Bic gas by measuring the grams
of gas used, the temperature, the pressure and the volume the gas occupies. The weight of the gas is obtained by
weighing the Bic lighter before and after use. The volume is determined by water displacement and the temperature
of the gas is assumed to be the same as the temperature of the water through which it passes.
Calculating the pressure of the gas is slightly more complicated. As the gas passes through the water it
becomes saturated with water vapor which contributes to the total pressure of the gas. To obtain the pressure of
just the Bic gas we can use Dalton's Law of Partial Pressures which states that, in a mixture of gases, each gas exerts
a partial pressure proportional to its concentration. Mathematically this is stated as:
Ptotal  Pwater  Pbic
The partial pressure of water can be obtained from the table of values in this experiment and is only dependent
upon the temperature at which the experiment is performed. The total atmospheric pressure is obtained from a
barometer and will be provided for you. Finally you can calculate the pressure of the Bic gas by subtracting the
pressure of water from the total atmospheric pressure.
8-1
You now have all the experimental information that you need. The molar massof the gas is calculated by
substitution into the equation below. Since you measured the volume of the gas in mL and the pressure in Torr, we
will use a gas constant R that contains these dimensions.
MW 
gRT grams of gas used 62,400 mL Torr/mole K Temperatur e 

Pbic Volume of gas 
PV
PART II: Freezing Point Depression
Have you ever wondered how antifreeze lowers the freezing point of water in the cooling systems of automobiles?
In the latter part of the nineteenth century the French chemist Francois Raoult noted that the vapor pressure of a
solvent was lowered by the addition of a solute (Raoult's Law), and that the freezing points of solutions also were
lowered.
Eventually, the following mathematical expression that related freezing point depression and molality (moles of
solute per kilogram of solvent) was developed:
ΔT = Kf msolute i
where:
ΔT = freezing point depression in °C
Kf = molal freezing point constant for the solvent in °C•kg/mol
m = molality of the particles in the solution in mol/kg
i = van’t Hoff factor
ΔT = Tsolution – Tpure solvent
Kf water = -1.86°C•kg/mol
The van’t Hoff factor, i, for electrolytes is equal to the number of ions present in the compound. In the case of a
non-electrolyte, the molality of the non-electrolyte and the molality of particles in solution are the same (a 1:1 ratio).
Automobile manufacturers make use of the principle of freezing point depression to protect engines from freezing
in cold weather. Antifreeze, a non-electrolyte, is added to the water-filled radiator that cools the engine. Under
most conditions the presence of the antifreeze molecules in the water is sufficient to keep the system from freezing.
In this investigation, you will determine the freezing point depression of antifreeze solutions by cooling them in an
ice-salt bath. You will also use this information to find the molar mass of ethylene glycol (C2H4(OH)2).
Molar Mass = grams of antifreeze per 1000 grams of solvent /molality of solution
PROCEDURE
PART I:
1. Obtain a ring stand, utility clamp, 250 mL volumetric flask and a water trough. Fill the water trough about
half full with tap water and the 250 mL volumetric flask to the etched line with tap water. You may find it
useful to use your wash bottle to add the last few mL of water to the flask. Place your thumb over the
opening of the flask and invert the flask so that the opening is below the water level in the trough. Clamp
the flask in place so that there is room to insert the gas tube between the flask and the bottom of the trough.
Be careful not to let any of the water leak out of the flask! If this occurs, you must start over.
2. Obtain one of the modified Bic lighters. Weigh the Bic lighter on the balance to two decimal places and
record its value on the Report Sheet.
8-2
3. Place the lighter below the opening in the neck of the flask. Turn the lighter on and collect the gas in the
flask until the water level reaches the neck of the flask. At this point, remove the lighter, blow it dry and
weigh the Bic lighter again. The weight of the gas used is calculated by the difference between the two
weighings.
4. The volume of the remaining water is determined by placing your thumb over the opening of the 250 mL
volumetric flask, removing the flask from the trough and pouring the water into a 100 mL graduated
cylinder. The volume of the gas collected is calculated by subtracting the volume of water remaining from
the initial volume of water used, which in this case is 250 mL.
5. Finally measure the temperature of the water in the trough and convert it to the Kelvin scale. Knowing the
temperature of the water you can determine the Pwater from the chart on page 3 and calculate the pressure
of the Bic gas by subtracting it from the atmospheric pressure.
6. When you have finished one trial, repeat the experiment again.
PART II:
1. Half fill a 600mL beaker with ice and cover it with about 20 grams of sodium chloride (NaCl). Stir this ice
salt mixture with a stirring rod until it reaches a constant temperature at or below -10°C. Cover the outside
of the beaker with aluminum foil with the reflective side in.
2. Half fill a test tube with distilled water and place it in the ice bath you created. Rinse the stirring rod and
use it to stir the water gently until ice crystals first appear. Use a thermometer to measure the freezing point;
record this value in the Data Table.
3. Find the mass of a 250mL beaker. Leaving the beaker on the balance, add exactly 10.00g of antifreeze from
a 10 or 25mL graduated cylinder and a dropper. Add 100mL of distilled water to the beaker and stir
thoroughly with a stirring rod. Label the beaker solution 1.
4. Repeat step 3 using 20.00g of antifreeze. Label this beaker solution 2.
5. Half fill a test tube with Solution 1 and place it in the ice/salt bath. Rinse the stirring rod and use it to stir
the solution until the temperature is 0°C. Add a small chip of ice to the solution. (This will eliminate the
possibility of supercooling.)
6. Repeat step 5 with solution 2.
7. Dispose of antifreeze solutions in the waste container provided.
Vapor Pressure of Water for PART I
Temperature
(K)
281
282
283
284
285
286
287
288
289
290
Pressure
( Torr )
8.0
8.6
9.2
9.8
10.5
11.2
12.0
12.8
13.6
14.5
Temperature
(K)
291
292
293
294
295
296
297
298
299
300
Pressure
( Torr )
15.5
16.5
17.5
18.6
19.8
21.0
22.4
23.7
25.2
26.7
8-3
Temperature
(K)
301
302
303
304
305
306
307
308
309
310
Pressure
( Torr )
28.3
30.0
31.8
33.7
35.7
37.7
39.9
42.4
44.6
47.1
Temperature
(K)
311
312
312
314
315
316
317
318
319
320
Pressure
( Torr )
49.7
52.4
55.3
58.3
61.5
64.8
68.3
71.9
75.7
79.6
Name
REPORT SHEET
MOLAR MASS DETERMINATION OF GASES & SOLUTIONS: Flick Your Bic & Freezing Point Depression
PART I:
Trial 1
Trial 2
Solution 1
Solution 2
Initial mass of Bic lighter
Final mass of Bic lighter
Grams of gas used
Initial volume of water in flask
Final volume of water in flask
Volume of gas collected
Temperature of water in °C
Temperature of water in Kelvin
Atmospheric Pressure (Ptotal)
Partial pressure of water (Pwater)
Pressure of Bic Gas (Pbic)
Theoretical Molar Mass of Butane (C4H10)
Experimental Molar Mass of Bic gas (Butane)
Percent Error
PART II:
Freezing point of distilled water in °C
Mass of Beaker in grams
Mass of Antifreeze in grams
Volume of water in mL
Mass of water in grams
Freezing Point in °C
Freezing Point Depression in °C
Molality, m, in mol/kg
Number of grams of antifreeze per 1000 grans of solvent
Theoretical Molar Mass of Antifreeze (C2H4(OH)2) in g/mol
Experimental Molar Mass of Antifreeze in g/mol
Percent Error
8-4
POST-LAB QUESTIONS:
PART I:
1. How can you explain the fact that the Bic fuel is a liquid in the lighter, but a gas when it is collected?
2. If some of the gas bubbles were not collected in the volumetric flask during the experiment but rather
escaped to the atmosphere, would the calculated molar massof the gas be greater or less than the
theoretical molecular weight? Explain!!
3. If the lighter was not dried properly after filling the flask and had some water on it for the second weighing,
would the calculated molar massof the gas be greater or less than the theoretical molecular weight?
Explain!
PART II:
1. What effect on the freezing point depression of water would a 1molal solution the ionic substance
(NH4)3PO4 have?
2. What is the density of distilled water assumed to be in this experiment? How would the answer differ if
the density were greater?
8-5