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PURPOSE : In this particular laboratory experiment, we are going to determine the molar mass of an unknown gas, from the gas density. We are also going to use gas density, along with the kinetic molecular theory, to explain the behavior of gas filled balloons. Finally, we are going to demonstrate the effects of temperature and pressure changes of gasses.
PROCEDURE : We are using the experimental procedure as described in our laboratory manual.

University of Wisconsin - River Falls
Laboratory Manual
Chemistry 116

Experiment 116-8
Molar Mass From Gas Densities
Pages 81 & 82

There are no changes to this laboratory or from the indicated laboratory procedure.
MATERIALS and APPARATUS :
Part A-1: In this part of the experiment, we are going to observe three balloons filled each with one of three different gasses. So as part of our materials we need to have three balloons as well as the three gasses. These balloons were already prepared for us and we kept underneath the fume hood for observation.

Helium Sulfur Exofluoride
He SF6
4.002602 g. 146.06 g.
Laboratory Stock Laboratory Stock


Ammonia
NH3
17.031 g.
Laboratory Stock

Part A-2-5c: During this part of the experiment, our materials and equipment consisted of one, clean, dry, 500mL plastic bottle with an airtight cover. Along with the bottle we need an electronic balance, a thermometer set up to measure the air temperature, as well as a barometer with a chart on how to interpret the readings. Finally we must have the six different gasses with which to do our experiment.



Distilled Water Helium
H2O He
33.00674 g. 4.002602 g.
Laboratory Stock Laboratory Stock


Oxygen Argon
O2 Ar
31.9988 g. 39.948 g.
Laboratory Stock Laboratory Stock


Carbon Dioxide Propane
CO2 CH3CH2CH3
44.0098 g. 44.09652 g.
Laboratory Stock Laboratory Stock


Natural Gas
CH4
16.04276 g.
Laboratory Stock
Part A-6: In this particular part of the experiment we made two different balloons. Therefore we need the balloons themselves, as well as the two different gasses that are going to go into the balloons. One of the balloons needed to be filled with Helium and then the other one with Carbon Dioxide. Then the balloons were to be taken back to our place of residence for observation.

Helium Carbon Dioxide
He CO2
4.002602 g. 44.0098 g.
Laboratory Stock Laboratory Stock
Part B-1: For this part of the experiment we needed a balloon filled with air. We also tried this part of the experiment when the balloon was filled with two other gasses. A total of three more balloons will be needed for this lab then. We also needed some liquid nitrogen for this experiment. In addition to the liquid nitrogen, we need a pair of tongs to remove the gas filled balloon from the nitrogen.

Helium Carbon Dioxide
He CO2
4.002602 g. 44.0098 g.
Laboratory Stock Laboratory Stock






Nitrogen
N2
28.01348 g.
Laboratory Stock*

** Please note that Liquid Nitrogen is at an extreme temperature of
77 K. This will burn skin as well as freeze most objects it comes
into contact with. Take extreme caution.
Part B-2: For this part of the experiment, we needed a large water bath full of cold water. Additionally we need to have brought an empty soda can with us to class. ( I prefer Coca-Cola myself ) Then, on a ring stand over the wire gauze, we heated up ten milliliters of water in the can over a Bunsen burner. The goal was to turn all of the water to steam and keep as much as possible within the can. After the water had all turned to steam, we used the lab tongs and inverted the can in to the cold water, very quickly.
Distilled Water
H2O
33.00674 g.
Laboratory Stock
Part B-3: This final part of the experiment was demonstrated by our instructor. Our instructor used a bell jar and three different balloons filled with air, helium and one with water, to show the effects that the atmosphere around us has on different gasses and on water.
OBSERVATION and DATA :
Part A-1: In the table below, there are recorded observations from the three different balloons that were placed under the hood for us to observe. There were three different balloons, each filled a different gas.

He balloon feels light, the balloon rises in our atmosphere, there is a medium pressure
consistency feeling to the outside of the balloon

SF6 the balloon feels very full and tight, compared to the others this balloon feels very
heavy, this balloon feels like it would not float as easily as the rest of the balloons.

NH3 of all the three, this appears to be the lightest and most thin pressure consistency,
feels most light but does not respond to the atmosphere as well as the others
TABLE-1



Part A2-5a: During this part of the experiment, all we did was measure the densities of the six gasses listed in the table below. To do this however, we needed to find the masses of each of the gasses, as well as the volume of the 500mL bottle. The bottle itself turned out to be 526mL, instead of 500mL.

The following table below, lists the mass of six different gasses. We measured the gasses on an electronic balance. Note, that the masses below include the mass of the bottle.
Mass of Gasses

O2 55.5811 g.

He 55.0005 g.

Ar 55.7398 g.

CO2 55.8329 g.

Natural Gas 55.2622 g.

Propane 55.5986 g.
TABLE-2

Part A-6: As I observed my two balloons, one filled with Helium and then the other filled with Carbon Dioxide for the 24hr. period, I noticed the both of the balloons leaked some of their gas. However, I also noticed that the balloon filled with Helium lost more gas than the balloon filled with Carbon Dioxide.
Part B-1: During this part of the experiment, we placed a balloon filled with a gas into the pool of liquid nitrogen. Our balloon was filled with CO2. The balloon shrank rapidly when it was placed in the nitrogen, but then when it was removed, it expanded again. While the balloon was expanding I noticed the formation of white condensation from inside the balloon. To my knowledge, the observation I had stumbled upon, was the formation of dry ice, as it was created when the balloon was submerged in the nitrogen.
Part B-2: In this part of the experiment, we observed the contraction of a gas, when it is transformed from a gas to a liquid, very rapidly. When we placed the heated can full of steam in the cold water bath, the can imploded trying to condense the whole structure into water again.
Part B-3: This final part of the experiment was demonstrated by our instructor. Inside the bell jar, we observed the helium balloon as well as the air balloon get larger as the air that was surrounding the balloon was removed from the chamber. As for the balloon filled with water, we did not notice as much of an extreme change, but it also seemed like it grew in size a small bit, but it also shrank while it was depressurizing too. The helium balloon posed the most unique reaction, by not floating one the bulk of the air was removed from the bell jar.
RESULTS and CALCULATIONS :
Part A-1: From the observations listed in TABLE-1, and if we look at the molar mass in table three below, we can determine that Helium is the lightest of the three gasses in this part of the experiment. My observations in TABLE-1 led me to believe that the ammonia (NH3) was lighter. This just goes to show that the different chemical composition of a gas, can be difficult to tell by simple observation. So, from the table below, we can see that ammonia is heavier than helium and that the sulfur exofluoride is heavier than both of the other gasses.
He 4.002602 g.
NH3 17.03056 g.
SF6 146.0564 g.
TABLE - 3
Part A2-5a: The data that we collected from this experiment is listed previously in TABLE-2. Now we are going to examine the data and do some calculations because our goal is to find the density of the six different gasses. To find the density of the gasses we first need to find the mass of each gas. In order to find the mass of each gas, we need to find the mass of the bottle without any air in it. In order to do this, we need to find the density of air, and then multiply it by the volume of the bottle. We can find the density of the air by reading the barometer and thermometer, and then finding the table in the CRC Handbook, that will tell us what the correct density of air is. The reading on the barometer was 731.3 torr and the thermometer read 22.3 degrees Celsius. When we looked up in the CRC Handbook, it told us that the density of air is .00115. Then we used this value and multiplied it by the volume of the bottle and we came up with the mass of the air in the bottle equaling .604g. Therefore the bottle itself weighed 54.7244g. Now the next step is to determine the mass of each gas. After the mass of the gas is found, we then can divide the mass by the volume of the bottle to find the correct density of the gas also. The table below (4) shows the mass as well as the density of each gas. These principles are also shown on the following graph.
Mass of Gas Density of Gas
O2 .8567 .00163
He .2761 .00052
Ar 1.0154 .00193
CO2 1.1085 .00211
Natural Gas .5378 .00102
Propane .8742 .00166
TABLE-4
Part A5b: Using the graph that follows, you will also see, outlined in pink, a prediction of the density of ammonia. We used the molar mass of ammonia an incorporated it with the rest of the gasses on the graph. The density of SF6 is too large to fit on the graph that is given, but if you use the algebraic equation for slope (y-x) / (y-x), we can find out an approximate density for the gas. To determine the slope you just use the values for any two points that you know for sure on the graph. (We are omitting the possibility of propane as a choice for one of the points, see part A5c for explanation.) The slope of the main line on the graph, indicated in yellow, is 1/2. The slope is a nice proportional number so we can easily take the molar mass of SF6 and divide it by three, take that value and compare it to our graph to find a density, then multiply that density by three. This will give us an approximate density for SF6 of .00858.
Part A5c: As indicated in the lab manual, propane does not fit in with the other values when we look at them on the graph. Propane is sometimes sold with another lighter gas included in it. We apparently have got some of the "cheap" propane. If we assume that natural gas is the other component, we can then determine how much really is propane and how much is natural gas. By doing a proportional calculation. We determined that 36% of the propane was actually methane, and 64% was really propane. To calculate this we needed to use the molar mass of propane as well as methane to do the conversion.