The Ideal Gas Law experiment focuses on determining the molar mass of a gas using the ideal gas law equation, PV=nRT. Students will measure pressure, volume, and temperature to calculate the number of moles of lab gas, specifically natural gas used in laboratory burners. The experiment emphasizes practical applications of gas laws in stoichiometry and includes safety protocols for handling materials. Ideal for chemistry students, this hands-on experiment enhances understanding of gas properties and calculations.

Key Points

  • Explains the ideal gas law and its application in determining molar mass.
  • Includes step-by-step procedures for measuring gas pressure, volume, and temperature.
  • Covers safety protocols for handling lab gases and equipment.
  • Provides calculations for determining the density of lab gas and air.
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Biology
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EXPERIMENT 7
GAS LAW APPLICATION
INTRODUCTION
This experiment involves making relatively simple measurements that provide data for
calculations using the ideal gas law to determine the molar mass of a gas. The principle
objective is to help to understand and practice these calculations so that calculations
involving amounts of gases can be used to describe gases as well as use their amounts in
stoichiometry.
The ideal gas law has several forms. The principle form is
PV=nRT
where P is pressure, V is volume, n is the number of moles of gas, R is a constant and T is
the absolute temperature. When the units of pressure, volume and temperature are
atmospheres, liters and Kelvins respectively, the value of R is 0.0821 L-atm/mol-K. Since
our objective is to find the molar mass, we need to separately determine the mass and
number of moles in a sample. We will obtain the mass by weighing and the number of
moles will be found indirectly: by finding the pressure, volume, and temperature of the gas
sample and solving the gas law for n.
The sample gas will be the natural gas used as the fuel for burners in the lab. We will
refer to it as "lab gas" for short. You will determine its molar mass and density. The pre- lab
questions provide additional practice exercises in the use of the ideal gas law.
MATERIALS AND SAFETY
No special hazards are involved in this exercise. Nevertheless, the use of safety goggles is
required due to the presence of other potentially hazardous materials in the lab area. Take
care with the thermometers to avoid placing them where they might roll off a bench top or
otherwise be vulnerable.
7-1
PROCEDURE
Take a clean, dry 250 mL Erlenmeyer flask from your drawer to serve as the gas container.
Obtain a thermometer and a rubber stopper (#6 size for most of these flasks) from the
equipment cart or other source identified by your instructor.
Place the stopper in the mouth of the flask, effectively enclosing a volume of air inside.
Weigh the flask, air and stopper together on an electronic balance and record the mass to
the nearest 0.001 g in the data section of the report. Record the air temperature in the
room. We will assume the air is at the same temperature.
Next, go to the hood area of the lab. There will be a set of hoses attached to the burner gas
lines with pinch clamps to control the flow of gas. The hoses will be attached to glass tubes
passing through rubber stoppers. Insert one of these tubes into your flask loosely and
release the pinch clamp so that gas can flow into your flask and flush out the air. You
need to keep the stopper loose in the flask so that air can escape. It is preferable to hold the
flask upside down for this operation.
After about two minutes of replacing air with lab gas, take out the filling tube and quickly
replace it with the original solid rubber stopper. Then replaced the pinch clamp to stop the
gas flow. Record the air temperature in the hood work area. We will assume that the lab
gas temperature is the same. Weigh the flask with gas and stopper on the electronic
balance and record the result. The mass should be less than when the flask was weighed
with air inside. If not, repeat the gas filling and weigh again.
To find the volume of the air and lab gas samples, fill the flask to the top with tap water
then insert the stopper so that excess water is pushed out and no significant amount of air
is trapped under the stopper. Dry off the overflowing water. Next, pour all of the
contained water from the flask into an empty graduated cylinder and measure the total
volume.
The pressure of the air and lab gas samples will be assumed to be the same as the room air
pressure. This can be found with the lab's barometer, hanging near the stockroom window
in the 103 room. The barometer has a centimeter scale on the outside. Read position of
the indicator against this scale and multiply by ten to obtain the pressure in millimeters of
mercury. Record the adjusted value.
Return the stopper and thermometer to the cart.
7-2
CALCULATION NOTES
We intend to find the mass of the lab gas by subtracting the mass of the empty flask and
stopper from the mass of the flask, stopper and lab gas. However, since the first weighing of
the flask included a mass of air, we need to estimate the mass of this air and subtract it. If we
assume that the air is dry (does not contain water vapor), we can use the chart below to find
its density. Use the closest temperature row and pressure column to find the value.
With this density and the volume of the flask, you can calculate the mass of the air that
occupied the flask.
While solving the ideal gas law for the number of moles of gas present in your sample, it is
important to be certain that the units of pressure, volume, and temperature are consistent
with the values of R used. The calculations section of the report will prompt you to convert
to appropriate units.
DENSITY OF DRY AIR (in grams per liter)
Pressure in mmHg
Temp in ° 720
730
740
750
760
770
15 1.161
1.177
1.193
1.210
1.226
1.242
16 1.157
1.173
1.189
1.205
1.221
1.238
17 1.153
1.169
1.185
1.201
1.217
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FAQs

How is the molar mass of the gas calculated in this experiment?
The molar mass of the gas is calculated using the ideal gas law equation, PV=nRT. First, students measure the pressure, volume, and temperature of the gas sample. From these measurements, they calculate the number of moles (n) of the gas. The mass of the gas is determined by weighing the flask before and after filling it with gas, allowing for the calculation of molar mass by dividing the mass by the number of moles.
What safety precautions should be taken during the experiment?
Safety precautions include wearing safety goggles to protect against potential hazards in the lab. Students should handle thermometers carefully to prevent breakage. Additionally, proper ventilation is necessary when working with lab gases to avoid inhalation of harmful substances. Following all lab protocols and instructions from the instructor is crucial for ensuring a safe experimental environment.
What is the significance of using the ideal gas law in this experiment?
The ideal gas law is significant in this experiment as it provides a reliable method for relating the physical properties of gases. By using the equation PV=nRT, students can understand how pressure, volume, and temperature interact to determine the behavior of gases. This foundational knowledge is essential for further studies in chemistry and real-world applications in fields such as engineering and environmental science.
What measurements are necessary to perform the experiment?
Necessary measurements for the experiment include the mass of the flask with air and then with lab gas, the temperature of the air and lab gas, and the volume of the flask. Additionally, students must measure the atmospheric pressure using a barometer. These measurements are crucial for accurately applying the ideal gas law and calculating the molar mass and density of the gas.

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