MOLAR MASS OF A GAS DETERMINED EXPERIMENTALLY
Reminder – Goggles must be worn at all times in the lab!
PRE-LAB DISCUSSION:
In this particular lab, we are going to try to determine the molar mass of butane, C4H10,
experimentally. A simple calculation using a periodic table would give us the correct answer for
the molar mass of butane, but you are going to see how close you can come to this accepted
value in the lab. From our studies in Chapter 11, we know that (PV)/(nT) = R rearranging to
solve for n, the number of moles:
n = (PV)/(RT)
We are trying to find the molar mass, which has units of grams/mole. We will measure the mass
of the butane released from the pressurized container, and we will measure the volume of the
gas that we collect at room conditions by water displacement. We merely need to plug in our
conditions of volume, temperature, and pressure into the ideal gas law in order to find n, the
number of moles of butane collected. Once we know the mass, and the number of moles, we
will divide the mass, m, by the number of moles, n.
Butane is a flammable gas, and at NO TIME during this lab should there be any use of an
open flame or other heat source!
PURPOSE:
The purpose of the lab is to experimentally determine the molar mass of butane.
PROCEDURE:
1. Determine an initial mass for the butane lighter provided by your instructor. As always,
remember to record a mass to two places past the decimal.
2. Set up the water bath for collecting gas. Your instructor has set one up as a demonstration
model. Filling the graduated cylinder with water at the beginning of the experiment can be
difficult. Try to have almost no bubbles before doing the procedure.
3. Keeping the cylinder upside down, put the lighter in the water below the cylinder.
4. Release butane from the pressurized container until the graduated cylinder is NEARLY
emptied of water but not past the last measurement tick. Be sure to keep the butane lighter
UPRIGHT as you dispense the gas into the collecting bottle! If you accidentally empty the
entire graduated cylinder of its water, you will have to repeat the entire procedure beginning
with re-weighing the butane lighter.
5. Once you have measured the volume of gas collected in the bottle, move the bottle up or
down so that the water level inside the bottle and outside the bottle are the same (be careful
not to raise the mouth of the bottle above the water level). THIS IS VERY IMPORTANT! If
you don’t do this simple step, the pressures inside and outside the bottle are not equal, and
you will have incurred an avoidable source of error.
6. Read the measure the volume of the gas collected in the graduated cylinder. Record the
butane volume in the Data section.
7. Using a thermometer, take the temperature of the water in the trough and record the value
to one decimal place.
8. Get the barometric pressure in the lab in mmHg (torr) and record this
value.
9. Reweigh the pressurized butane lighter, recording its new mass
OBSERVATIONS AND DATA:
Mass
1. Beginning mass of butane lighter ___15.5______________grams
2. Final mass of butane lighter ___15.43______________grams
3. m = Mass of butane collected (line 1 – line 2) ___.07______________grams
Volume
4. TOTAL volume of the graduated cylinder ___50______________mL
5. Volume of water remaining after gas collection __46_______________mL
6. Volume of butane collected (line 4 – line 5) ____4_____________mL
7. V = Volume of butane, converted to liters (line 6 / 1000) ____0.004_____________L
Temperature
8. Temperature of water bath _____23____________°C
9. T = Temperature of water in (line 8 + 273.2) _296.2________________K
Pressure
10. Barometric pressure __29.96_______________inHg
11. Convert barometric pressure to mmHg (line 10 * 25.4)
_760.984________________mmHg
11. Vapor pressure of water at the above temperature ___21.068______________mm Hg
(see page 899 of your chemistry book)
12. Pressure of dry butane (line 10 – line 11) __731.024_______________mm Hg
13. P = Pressure of dry butane in atmospheres (line 12 / 760)
_.96187368421________________atm
CALCULATIONS: Show your work! Observe significant figures and include units.
1. Calculate the number of moles of butane collected, using n = (PV)/(RT)
R= 0.0821
T= 296.2
V= 0.046
P= .962
All= 1.076
2. Calculate the experimental molar mass of butane by dividing the mass(m) by n, the number
of moles.
0.07 g/1.076 = 0.65
3. Calculate the accepted value for the molar mass of butane using a periodic table.
48.04 + 10.1 = 58.14
4. Calculate the ABSOLUTE ERROR, the difference between the accepted value and
your experimental value.
58.15/65 x 100 = 89.46
5. Calculate the percentage error in your result.
(65-58.14)65 x 100 = 89.46
QUESTIONS FOR DISCUSSION:
1. List at least THREE sources of EXPERIMENTAL error (NOT calculation error).
Be specific!
Bubbles are in the gas
Gas is lost while the butane lighter is on
The cylinder is not straight
2. How many grams of butane did you collect in this experiment? What volume would the
same number of grams of ethane (C2H6) have occupied at the same conditions of
temperature and pressure?
0.07 grams
24.02 + 6.06 = 30.08
30
V=RnT/P
.0821 x 30.08 x 294.2/ .96187368421 = 755.35 ml