Determination Of The Molar Mass Of A Volatile Liquid 82l27

1

Determination of the Molar Mass of a Volatile Liquid Banda, Marybeth Hope T. Abstract: In this experiment the molar mass of a volatile liquid was determined. The molar mass was calculated by measuring the temperature, pressure, and volume of the gas. Experimental results show that the molar mass of the volatile liquid is 49.64 g/mol. This result is higher than the actual value of the molar mass which is 46.08 g/mol. Introduction The determination of the molar mass of a volatile liquid is important in the classification of newly synthesized chemical compounds. The method used in this experiment is one of the simplest method in the determination of the molecular masses of chemical compounds. The working concept of the method used in the experiment is the ideal gas law: PV=nRT. If the pressure, volume, and temperature of the gas can be measured, then the amount of gas can be calculated. Further, if the mass of the gas can be measured, then molar mass of the gas can also be calculated:

beaker. The water was heated to boiling. After wrapping the flask with aluminum umbrella and Teflon tape, the flask was then immersed in the boiling water up to its neck until no Schlieren pattern was observed and until no vapor was observed to evolve from the flask. The evolution of vapor from the flask was tested by hovering a clean, dry watch glass over the pinhole in the aluminum cover to check for condensation. Meanwhile, the temperature of the water bath was recorded. The flask was heated further for two minutes. This was done to permit the excess vapor to escape and to equalize the pressure between the interior and exterior of the flask.

The objective of this experiment is to determine the molar mass of a volatile liquid. Experimental Methods The materials used in this experiment are: iron clamp, aluminum foil, boiling chips, iron ring, hot plate, thermometer, thermometer holder, Teflon tape, 125 mL Erlenmeyer flask, syringe, 400 mL beaker, and iron ring. The molar mass of the volatile liquid was determined by measuring the temperature, pressure, mass, and volume. The atmospheric pressure was recorded. An Erlenmeyer flask covered with aluminum foil was weighed. Then, three milliliters of the unknown solution was injected through the aluminum foil. The sides of the flask was wrapped with an aluminum umbrella and Teflon tape. Meanwhile, water and some boiling chips were added to a

Figure 1. The Set up After which, the flask was removed from the boiling water and was allowed to cool to room temperature before running tap water along its exterior. The flask was dried and then weighed. Two trials were made. After the trials, the flask that was used was filled with water up to its neck and then weighed.

2 2 and the corresponding masses. The mass of gas was obtained by subtracting the mass of flask and foil from the mass of the flask and foil with gas sample.

Results and Discussion The molar mass of a volatile liquid can be calculated when its pressure, volume, temperature, and mass can be measured. According to the equation:

Table 3. Data for Molar Mass of Volatile Liquid Parameters Temperature of water bath (K) Atmospheric Pressure (atm)

Trial 1

Trial 2

366.15

367.15

0.975

0.975

Table 1. Weighing Data For Volume Of The Flask

Mass of gas (g)

0.2526

0.2016

Parameters Mass of flask with foil cover (g) Mass of flask filled with water and covered with foil (g) Mass of water (g) Density of water (g/mL)

Volume of flask (mL)

141.1511

141.1511

Molar Mass (g)

55.14867

44.13435

Data 78.0689

Table 2 shows the data that was used in the calculation of the molar mass of the volatile liquid and the calculated molar mass for Trials 1 and 2. The molar mass of the gas of the volatile liquid was calculated as shown below:

219.22 141.1511 1.00

Table 1 shows the weighing data for the calculation of the volume of the flask. The mass of water was obtained by subtracting the mass of the flask with foil from the mass of the flask with foil filled with water. The volume of the flask was then computed by multiplying the mass of water with the density of water as shown by the calculation below:

For Trial 1

(

)( (

⁄

)(

)(

) )

For Trial 2

As the unknown test liquid was evaporated, it displaces the air molecules in the flask and fills the flask with its own molecules. A change in the mass of the flask was then observed. This mass difference is the mass of the gas molecules. Table 2. Mass of gas Parameter Mass of flask, foil, and gas sample (g) Mass of flask and foil (g)

Trial 1

Trial 2

78.3215

78.2705

78.0689

78.0689

Mass of gas (g)

0.2526

0.2016

Table 2 shows the data that was used in the computation of the mass of the gas for Trials 1 and

(

)( (

⁄ )(

)(

) )

The mean molar mass was calculated and was found to be 49.64 g/mol; as shown on the calculation below: ̅

The identity of the unknown test solution was revealed to be ethanol; with a molar mass equal to

3 46.08 g/mol. The percentage error was calculated as follows: |

| |

|

This error in the determination of the molar mass is attributed to the high altitude of the locality where the experiment was conducted. This lowers the pressure and consequently results to a higher molar mass than expected. Conclusion and Recommendations Experimental results show that the molar mass of the volatile liquid is 49.64 g/mol. This calculated molar mass is 7.729% higher than the actual molar mass of the gas which was ethanol. For future conduct of the experiment, the author recommends that more Trials be made to increase the accuracy of the experimental results.

References 15 Molar Mass of a Volatile Liquid. (n.d.). Retrieved from www.lexington.k12.oh.us/hs/strickler/co ncrete/index.php/...file/.../85/ Determination of the Molar Mass of a Volatile Liquid by Vapor Density. (n.d.). Retrieved from http://www.chemtopics.com/aplab/mmv liq.pdf Determining the Molar Mass of a Vapor. (n.d.). Retrieved from Chemistry Teacher Resources: http://chemmovies.unl.edu/chemistry/d ochem/DoChem078.html Gelder, J. I., Gettys, N. S., & Eubanks, I. D. (n.d.). EXPERIMENT 8: MOLECULAR WEIGHT OF A VOLATILE LIQUID. Retrieved from AP Chemistry by Satellite: http://intro.chem.okstate.edu/HTML/SE XP8.HTM