Determination of MM from Vapor
Density
Introduction:
An archaeologist is planning a dig in Egypt. He only has enough time and money to digup a small portion of the dig site. The archaeologist needs to determine how best to allocate
his given resources to excavate the biggest area possible. Scientists go through the same process when they are attempting to determine the mass or volume of a gas, liquid, or solid. They
try to come up with the most efficient process to create accurate results so they can determine
things like vapor density or molar mass. Vapor density is the density of a gas when it is being
compared to hydrogen; it can be used to determine molar mass. Molar mass is the mass of one
mole of substance; this varies from element to element. This method of measurement for molar mass is effective and can be used when dealing with volatile liquids. A volatile liquid is a liquid that likes to vaporize and evaporate. If vapor density is accurately determined, then the
molar mass of a volatile liquid may be determined.
Materials and Methods:
The experimenters first obtained a 250mL Florence flask and then cleaned it. They proceeded to obtain a stopper with one hole, a piece of rubber tubing, and glass tubing; the glass
tubing was placed inside the rubber tubing then the glass tubing was threaded through the hole
in the stopper. This was placed in the flask’s opening and weighed. A hot water bath was setup and snow was obtained from outside for a cold-water bath. Five milliliters of isopropyl alco-
hol was added to the flask. The flask was heated up in the hot water bath over a Bunsen burner. About half-way through heating class ended and the flask with alcohol sat for four days.
The flask and hot water bath were reheated again this time using a hotplate until all the liquid
in the flask evaporated. The experimenters then carefully placed the flask in the snow to cool
down. After about five minutes they wiped down the flask then weighed it to determine mass
of the gas. They placed the flask back in the snow to finish cooling the flask down and to allow
the gas to condense. The flask sat overnight in the snow then it was weighed again.
Results:
Temperature of Water (Boiling)
98.6˚C
Barometric Pressure
770mmHg
Mass of Flask
157.17g
Mass of Flask and liquid after condensing
158.17g
Mass of Liquid after condensing
1.00g
Volume of Flask
250mL
Pressure: 770mmHg/760mmHg = 1.0132atm
Temperature: 98.6˚C + 273.15K = 371.75K
Constant R = 0.082057
=
=.0319mol
Molar Mass = 1.00g/.0319mol = 31.3g/mol
% error:
52%
Conclusion:
The experimenters determined the compound to have a molar mass of 31.3g/mol. In
reality isopropyl alcohol has a molar mass of 60.10g/mol. There was a percent error of 52%.
Many different factors could have contributed to this outcome; one being the apparatus itself.
The vapor had to go through a glass tube and rubber tubing before being allowed to escape;
this could have trapped some the vapor so when it condensed it dripped back into the flask rather than escaping. The original heating source was a Bunsen burner that created inconsistent
heat and when it was reheated a hotplate was used which created consistent heat. This improper heating could have led to mistakes in the results. Completion of the lab in three different days with three different pressures could have also led to skewed results.
In order to obtain better lab results a few changes should occur. The first change suggested would be the use of one type of heating source that gives off a consistent source of
heat. The next would be having enough time to finish the lab in one day including the time to
cool the flask down to allow all the gas to condense. Only using a glass rod as opposed to both
a glass rod and rubber tubing to divert the fumes might help with allowing more of the gas to
escape.
The experimenters have different results from other groups. One group had a minuscule number while others were around the experimenters’ results but many groups were not
completed at the time results were discussed.
Both culinary students and professional chefs alike are known for making some weird
concoctions and venturing into some bizarre and unknown territory. They come up with many
different recipes and sometimes only through trial and error will this recipe turn into a tasty
and delicious one. The chefs use different amounts of the ingredients and then they taste and
adjust the ingredients ratio accordingly.