Report 12: IDEAL GAS LAW
Title:
Ideal Gas Law
Laboratorial Report 12
Created to fulfill the assignment for Mechanic and heat EN222 subject
By:
Debby Syefira
Damavara
I Wayan Surya Aryana
Reyhan
Lecturers:
Lina J Diguna
Fatih
Sampoerna University
Performed: Jakarta, June 3th, 2015
Report 12: IDEAL GAS LAW
Abstract
The ideal gas law could be determined by varying method. One of them could be apply by
using the volatile liquid as the substance. Moreover, the pressure, volume, number of mole, and
time is related each other in the ideal gas law. In this experiment, the student is expected to be able
to use the ideal gas law to calculate the molecular weight of a volatile liquid compound. There are
numbers of equipment and procedures that should be prepared and followed by the student during
the experiment. The result of this experiment will be processes use the formula given by the theory.
There will be a deep discussion about the result of this experiment. There will be also list of
questions that will be answered in order to deeper the student analysis about this topic. At the end,
the student will come with the conclusion and some recommendation about this experiment in
order to provide a good input to improve the next experiment.
Report 12: IDEAL GAS LAW
CHAPTER I
OBJECTIVES
The objective of this experiment are:

Using the ideal gas law to calculate the molecular weight of a volatile liquid compound by
measuring the mass, volume, temperature, and pressure of the compound in its gaseous state
Report 12: IDEAL GAS LAW
CHAPTER II
METHODOLOGY
2.1 Materials
The materials that are being used in the experiment are listed in below:
Table 1. Materials
No
.
1
2
3
4
5
6
Material
No.
Material
600 mL beaker
150 mL Erlenmeyer flask
Graduated Cylinders (10 and 100 mL)
Thermometer
Electric Hot Plate
Ring Stand
7
8
9
10
11
Clamp or Iron Ring
Digital Balance
Aluminum Foil
Rubber Bands
Unknown Liquids
2.2 Procedures
1. Determine the combined total mass of a dry 125 mL Erlenmeyer flask, a rubber band, and
a square of aluminum foil, and record this mass on the report form. NOTE: If you need to
exchange your flask for any reason, be sure to re-weigh the new flask, as the weights of
even the same sized flasks vary considerably. Likewise, always use the same balance to
make all of your weighing.
2. Create a hot water bath by filling a 600 mL beaker half full of water. Heat to boiling on a
hot plate, and while waiting for the water to boil, use a 10 mL graduated cylinder to measure
and pour approximately 2 mL of your liquid unknown sample into the flask. Secure the
aluminum foil over the mouth of the flask with the rubber band. Make a tiny hole in the
foil with a pin to let excess vapor escape during heating.
3. Clamp the flask assembly into the beaker so that flask is as far down as possible in the
beaker. Heat at the boiling point of water until liquid is no longer visible in the flask.
Continue heating for another 10 minutes. Measure the temperature of the gas occupying
the flask by recording the temperature of the boiling water surrounding the flask to the
nearest ±0.1℃. Since the flask is even so slightly open to the atmosphere, the pressure of
the gas must be equal to the barometric pressure. Measure the atmospheric pressure with
Report 12: IDEAL GAS LAW
the barometer in the laboratory and record the current barometric pressure on the report
form.
4. Remove the flask from the water bath by loosening the clamp from the supporting rod and
moving it upward. Allow the flask to cool to room temperature, and dry it on the outside
gently and thoroughly with a paper towel. Weigh the flask along with its contents, the
aluminum foil and rubber band, and record this value on the report form.
5. Accurately measure the volume of the flask (it is more than 125 mL!) by filling it with
water all the way to the brim and measuring the volume of water with a 100 mL graduated
cylinder. Record the flask volume on the report form.
6. Calculate the moles of vapor from its pressure, volume, and temperature, and from this
value and the mass of the vapor, calculate the molar mass, or molecular weight, of the
vapor.
7. Be sure to return all equipment to its proper storage location.
Report 12: IDEAL GAS LAW
CHAPTER III
RESULT AND DISCUSSION
3.1 The Principles of Ideal Gas Law.
There are varying method to determine the molecular weight of a compound. One of the
method could be used for the volatile liquids. Volatile liquids are Volatile liquids have low boiling
point. Because of their low boiling points, volatile liquids convert to the gas phase at a lower
temperature and the gas molecules can diffuse faster than the molecules of a non-volatile liquid
(List: Volatile Liquids | Flipped Science, 2012). Most of these types of compounds will behave
like an ideal gas when converted to the vapor state. The ideal gas equation is:
pV = nRT
In this equation, P is the pressure of the gas, V is the volume of the gas, n is the amount of
the gas in moles, and T is the Kelvin temperature of the gas. R is called the ideal gas constant.
The value of R will differ depending on the units used for pressure and volume. When P is in
atmospheres and V is in liters, the value of R is 0.08206 (L atm) / (mol K). (EXPERIMENT 8 Ideal Gas Law, n.d.)
This equation is useful because it allows one to calculate the pressure, volume, temperature
or number of moles of a gas simply by knowing the other three variables and doing a little algebra.
In the following experiment you will use a setup with which you can easily determine the values
for pressure, volume and temperature of a gas. Once these values have been found, you can
determine the amount of the gas in moles, and from the mass of the gas in grams, you can calculate
the molar mass (molecular weight) of the gas as follows:
moles of gas = n =
PV
RT
and
molecular weight =
grams of gas
moles of gas
Gases, unlike solids and liquids, have neither fixed volume nor shape. They expand to fill
the entire container in which they are held. The pressure of a gas is defined as force per area. The
Report 12: IDEAL GAS LAW
standard or SI unit for pressure is the Pascal (Pa) which is the force exerted by the gas in Newtons
divided by area in square meters, N/m2. However, atmospheres (atm) and several other units are
also commonly used. (EXPERIMENT 8 - Ideal Gas Law, n.d.)
The ideal gas law assumes several factors about the molecules of gas. The volumes of the
gas molecules themselves are considered negligible compared to the volume of the container in
which they are held. We also assume that gas molecules move randomly, and collide in completely
elastic collisions. Attractive and repulsive forces between the molecules are therefore considered
negligible. (EXPERIMENT 8 - Ideal Gas Law, n.d.)
We can also use the ideal gas law to quantitatively determine how changing the pressure,
temperature, volume, and number of moles of substance affects the system. Because the gas
constant, R, is the same for all ideal gases in any situation, if you solve for R in the ideal gas law
and then set two terms equal to one another, you obtain a convenient equation called the combined
gas law:
R=
𝑃1 𝑉1
𝑛1 𝑇1
𝑃 𝑉
= 𝑛2 𝑇2
2 2
Where the values with a subscript of "1" refer to initial conditions and values with a
subscript of "2" refer to final conditions. (EXPERIMENT 8 - Ideal Gas Law, n.d.)
Report 12: IDEAL GAS LAW
3.2 Discussion 1: Determining the Molecular Weight of the Liquid
Result:

Mass of flask, aluminum foil, and rubber band = 0.080 kg

Temperature of boiling water = 99℃ = 372 K

Atmospheric pressure = 1 atm

Volume of flask = 192 mL = 1.92 L

Mass of flask, aluminum foil, rubber band, and condensed vapor = 0.080

Mass of condenser vapor ≈ 0.0001.

Moles of vapor =

Molecular weight of ethanol 70% =
Calculation:

Mole:
𝑃𝑉 = 𝑛𝑅𝑇
(1 𝑎𝑡𝑚)(0.0004 𝐿) = 𝑛(0.08206)(372 𝐾)
𝑛 = 1.3 × 10−5 𝑚𝑜𝑙

Molecular weight:
𝑛=
𝑚𝑎𝑠𝑠𝑎
𝑚𝑟
𝑀𝑟 =
𝑀𝑟 =
𝑚𝑎𝑠𝑠𝑎
𝑛
0.0001 𝑘𝑔
1.3 × 10 −5 𝑚𝑜𝑙
𝑀𝑟 = 7.692 𝑘𝑔/𝑚𝑜𝑙
Explanation:
Based on the result gotten, the mole of the experimented liquid is 1.3× 10−5 mol. This
value is basically gotten by calculating the variable pressure, volume, R, and temperature of
the system in the experiment. By finding the moles of the liquid, the molecular weight
consequently can be determined; by dividing the mass to the mole, the molecular weight is
7.692 kg/mol.
Report 12: IDEAL GAS LAW
3.3 Answering Questions
1) If the outside of the flask is not dried after vaporizing the liquid, will the unknown’s
calculated molecular weight is too high or too low? Explain!
Answer: by using the formula below:
𝑛=
𝑚𝑎𝑠𝑠𝑎
𝑚𝑟
It can be said that when the mass is increase, the molecular weight will also be increased.
Therefore, when the flask is not dried after vaporizing the liquid, the mass that is being
measured will increase, and the molecular weight will be too high.
2) If the volume of the vapor is assumed incorrectly to be 125 mL (from the label on the
flask) instead of the measured volume of the flask, which effect would this have on your
calculated molecular weight?
Answer: considering the volume of vapor is 0.4 mL in which it is far much lesser that the
assumption in the question, the molecular weight will be lower that it should be. It is
because, based on the formula below:
𝑃𝑉 = 𝑛𝑅𝑇
When the volume is higher than what it is should be, the moll will be increase in which it
is lead to the decreasing in the weight of the molecular; it is expressed by the formula of
𝑛=
𝑚𝑎𝑠𝑠𝑎
𝑚𝑟
3) If all unknown liquid does not vaporize in the flask during heating, will the calculated
molecular weight be too high or too low? Explain!
Answer: since the molecular weight that is being calculated is come from the formula of
𝑃𝑉 = 𝑛𝑅𝑇 and
Report 12: IDEAL GAS LAW
𝑛=
𝑚𝑎𝑠𝑠𝑎
𝑚𝑟
In which they need variable volume and variable mass, therefore when the liquid does not
vapor, in which it is mean there are no variables volume and mass, the molecular weight
cannot be determined.
4) Solve the following problem
a. How many moles of gas are contained in 890 mL at 21.0℃and 750.0 mmhg pressure?
b. At what temperature will 0.654 moles of neon gas occupy 12.30 liters at 1.95 atm?
Answer:
a.
𝑃𝑉 = 𝑛𝑅𝑇
(
750
) 𝑎𝑡𝑚 × 0.89 𝐿 = 𝑛 × 0.08206 × 294𝐾
760
𝑛 = 0.03640 𝑚𝑜𝑙
b.
𝑃𝑉 = 𝑛𝑅𝑇
1.95 𝑎𝑡𝑚 × 12.30 𝐿 = 0.654 𝑚𝑜𝑙𝑒𝑠 × 0.08206 × 𝑇
𝑇 = 446,920 𝐾
Report 12: IDEAL GAS LAW
CHAPTER IV
CONCLUSION
4.1 Conclusion

Mole of the liquid is 1.3 × 10−5 𝑚𝑜𝑙

Molecular Weight of the liquid is 𝑀𝑟 = 7.692 𝑘𝑔/𝑚𝑜𝑙
4.2 Recommendation

It is advisable if the apparatus used in the experiment is complete.
Report 12: IDEAL GAS LAW
REFERENCES
EXPERIMENT 8 - Ideal Gas Law. (n.d.). Retrieved July 4th, 2015, from
http://swc2.hccs.edu/pahlavan/intro_labs/Exp_15_Molecular_Weight_Determination_of_
Vapor.pdf
List: Volatile Liquids | Flipped Science. (2012, November 8th). Retrieved July 4th, 2015, from
FLIPPED SCIENCE: http://flippedscience.co/2012/11/08/nutshell-volatile-liquids/
Serway, R. A., & Jewett, J. J. (2014). Physics for Scientists and Engineers. Mary Finch.