Isabina Lloyd
AP Chemistry
Lab Report
Determination of the Molar Mass of Volatile Liquids
Abstract: This lab was conducted to calculate the molar mass of three volatile liquids by utilizing the ideal gas
law. The molar masses I came up with in the experiments did not match the literaure values, but it is a huge
error on my part.
Introduction: The concepts that this lab deals with is the molar mass and the ideal gas law. This law deals with
four measurable properties, those being pressure in atm, volume in liters, number of moles, and temperature in
Kelvin, which amounts to the equation PV = nRT. In order to find the molar mass of a volatile liquid, this
equation is arranged as molar mass (g/mol) = [mass(g) x RT(K)]/ [P(atm) x V(L)] This equation is important to
finding the molar mass of a volatile liquid. However, you would have to know the variables to use this equation.
R is a constant, which is equal to 0.082057 (1 L*atm/K*mol). The T is the temperature of the boiling water
used to heat up the pipets, in Kelvin. The mass is the mass of the volatile liquid vapor in grams. The pressure is
measured for the day in mm Hg, but is converted into atm. The V is the volume of the pipet in liters.
Materials & Methods: First I boiled 300 ml of water in a 400 ml beaker on a hot plate. While that was boiling,
I took three 15-ml jumbo Beral-type pipets, taking some pliers and scissors to them to form fine capillary tips, 1
cm long. I labeled each pipet from numbers one to three, then measured the mass with a milligram balance,
recording the mass to the nearest 0.001 g in my table. I then put about 1 ml of ethyl alcohol in each of the
pipets, and with some plastic tubing to hold them together, I lowered them into boiling water with a test tube
clamp, bulb first. They were kept in the water for five minutes before they were taken out of the boiling water
and dunked into the room temperature one. After drying them off, I weighed the condensed liquid in the pipets
on the balance and recorded the masses. After cleaning, filling them up with water completely and weighing the
pipets to get the mass of the water filled pipet, I repeated most of the procedure for acetone and isopropyl
alcohol.
Results:
Temperature of boiling water bath
100.1 Cº
Barometric pressure
760 mm Hg *
Temperature of room temperature water bath
21.6 Cº
Density of water at room temperature
0.8610 g/ml
Data Table 1 - Jumbo Pipets
Jumbo Pipet #1
Jumbo Pipet #2
Jumbo Pipet #3
Mass of empty pipet
1.121 g
1.125 g
1.142 g
Mass of pipet and water
6.746 g
6.778 g
6.847 g
Mass of water in filled pipet
5.625 g
5.653 g
5.705 g
Volume of pipet
6.533 g
6.566 g
6.626 g
Date Table 2 - Volatile Liquids
Trial 1
Trial 2
Trial 3
Ethyl Alcohol
Mass of pipet and condensed ethyl alcohol
1.144 g
1.150 g
1.162 g
Mass of condensed ethyl alcohol
0.023 g
0.025 g
0.020 g
100.594 g
109.341 g
87.473 g
Mass of pipet and condensed acetone
1.210 g
1.223 g
1.239 g
Mass of condensed acetone
0.089 g
0.098 g
0.097 g
389.254 g
428.617 g
424.243 g
Mass of pipet and condensed isopropyl alcohol
1.187 g
1.207 g
1.212 g
Mass of condensed isopropyl alcohol
0.066 g
0.082 g
0.070 g
Molar mass of ethyl alcohol
Acetone
Molar mass of acetone
Isopropyl Alcohol
Molar mass of ispropyl alcohol
288.660 g
358.638 g
306.155 g
*The barometric pressure may or may not of been the true value. I did this experiment on two different days,
and forgot to record the pressure, so I used this value to simplify calculations, so the molar masses of the
liquids may be way off.
Discussion: The molar masses I ended up with don't appear to be anywhere comparable to the actual literature
values of each of the volatile liquids, so I must have messed up big time somewhere. I might not of boiled the
pipets long enough because there must of been a significant amount of liquid left in the tubes in order to have
molar masses so high. I'm sure that I did the calculations correctly because of what I did in the pre-lab measured
fairly accurately, so my variables must have been somewhat askewed. I also might have put a little too much
volatile liquid in each tube by accident. Also, the pressure I used is not the true value because I forgot to record
it on both days. All of these contributed to my huge values that are untrue.
Conclusion: I learned in this lab that one must be conscious enough to record the data on the day/s you do the
lab, and that the data has to be adjusted for everything that could change, such as pressure or tempurature of the
boiling water. Working on a lab on my own is a fairly difficult experience if one is slightly forgetful as I am.