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CHM170L Physical Chemistry 1 Laboratory 4th Quarter SY 2009-2010

Determination of Molar Mass of a Volatile Liquid by Vapor-Density MethodNieva, Aileen D.1, Arceo, Mary Anne V., Cuales, Jelline C., Kim, Sung Min, Ngan, Emil Joseph T., Rivera, Jainie Lynne B.21Professor, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology; 2Student (s), CHM170L/A41, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology

ABSTRACTIn this experiment, the objective is to estimate the molar mass of volatile liquids from their vapor densities at a temperature above their boiling points using the Dumas method. In the Dumas method a volatile liquid is heated to a known temperature (above its boiling point) and allowed to escape from a container through a tiny orifice. Once the liquid has vaporized, the container is cooled to room temperature. Volatile substances are usually composed of non-polar molecules. Among five reagents available, three can be used namely acetone, ethyl acetate and ethyl alcohol. After introducing the sample with syringe in the flask, it should be boiled at least 10 minutes until the liquid is evaporated. The flask should be removed for cooling down and should be reweighed again. There are many factors which can be considered for the percentage errors. One of which is the intermolecular forces of each compound. Ethyl alcohol for example has the ability to form hydrogen bonds due to the presence of lone pairs of electrons in the oxygen atom behaving non-ideally. The molecular volume and the intermolecular forces are some of the variables which create deviations from the ideal behavior. Therefore conclude, among the three samples given, the ethyl acetate has the one with the highest value for molar mass of the vapor which is 95.634 percent for trial 1 and 96.677 percent for trial 2.

INTRODUCTIONOne of the properties that helps characterize a substance is its molar mass. Chemical and physical methods for determining atomic and molecular formula weights or molar masses have been important historically as a way of analyzing and categorizing new materials. The modern laboratory is generally equipped with instrumentation which makes many of these methods obsolete. However the principles upon which the older methods were based are not insignificant and many form the foundation for the prediction of physical and chemical properties and behaviors of substances.

Volatile substances are usually composed of nonpolar molecules. The classic Dumas method for determining the formula weight of a volatile liquid is a case in point. The Dumas method is one of the simplest ways to measure the molecular weight of a substance. Avogadro proposed as early as the mid-1800's that equal volumes of gases measured under identical conditions would contain equal numbers of gas particles. With an established relative atomic mass scale it was possible to describe a constant volume which would contain a gram-atomic weight of a gaseous element or compound under fixed conditions, what we know today as the molar volume. At STP this volume is 22.4 L for an ideal gas.

An understanding of the various gas laws allows for an application of this useful information to liquids and solids which are appreciably volatile, i.e., possess relatively high vapor pressures. As long as the temperature and pressure are known, a measured volume of gas can be converted to moles since: PV =nRT n= PV/RTMassing a sample of gas is relatively simple. These two pieces of information are the minimum required for a molar mass determination.

In the Dumas method a volatile liquid is heated to a known temperature (above its boiling point) and allowed to escape from a container through a tiny orifice. Once the liquid has vaporized, the container is cooled to room temperature. Gradually the vapor which remained in the container at the higher temperature condenses to a liquid and is then massed. If the volume of the container is known along with the high temperature, the room pressure can be used (because the system is open to the atmosphere through the orifice) to calculate moles. From there a molar mass can be determined.

This method depends on a lot of things going right. One assumption is that while the liquid is volatile enough to vaporize at the elevated temperature, it is not so volatile that a significant amount will be lost to evaporation through the orifice as the container cools. The vapor is also assumed to behave ideally at the temperature and pressure at which it occupies the container. The amount of error implicit in this approximation varies from compound to compound and is tied to the variables which create deviations from ideal behavior: molecular volume and intermolecular forces.Generally speaking, the larger these are, the greater the error in the determination. The situation is further complicated by the interaction of these two factors. For example, a small molecule may have significant intermolecular forces (hydrogen bonding perhaps) but a large molecule may have comparatively weak forces (like dispersion forces). Combinations run between these extreme examples making predictions of error from this source difficult. METHODOLOGYEXPERIMENTAL METHOD

Materials

Aluminium foil, fine copper wire, razor blades, pliers or wire cutter, boiling stones, syringe

Reagents

acentonitrile, ethanol, ethyl acetate, isopropyl acetate, 2-propanolEquipment and GlasswareAnalytical balance, 600-mL beaker, 125-mL Erlenmeyer flasks, Bunsen burner, wire gauze, thermometerSet-up of Apparatus

aluminum foil

Erlenmeyer flask

Beaker

Wire gauze

Bunsen burnerExperimental Procedure

A clean, dry 125 mL Erlenmeyer flask serves as a suitable container. Its mass is small enough for the analytical balance and large enough to contain a sufficient mass of vapor. The mass of the flask on the balance is required for the volume determination. When full of water, it will be too heavy for analytical balance.

A cap may be fashioned for the flask from a square of aluminium foil and secured with fine copper wire twisted tightly around the neck just below the rim. Excess foil should be removed as it provides a place for condensed water. The sample is introduced with syringe. The needle is used to make a tiny hole in the foil cap and 3-4 mL of the liquid is injected.

Enlarging the hole with an unsteady hand will introduce serious errors into the determination.

The flask should be immersed in a 600 mL beaker of water. The water level should be high enough to cover most of the flask but not so high as to allow water to enter through the hole in the foil. The water bath should be brought quickly to a boil but then the heat should be reduced to achieve a gentle boil for about 10 minutes or until all the liquid is evaporated. The flask is removed from the water and allowed to cool down. Water must be removed completely from the outside of the flask. When the flask has returned to room temperature and is completely dry on the outside, it should be reweighed. Additional trials may be done simply by adding more liquid through the same hole.

Repeat the procedure described above.

After the final trial, the cap assembly is removed and the flask is rinsed thoroughly and then filled almost completely with room-temperature water. Repeat the entire procedure stated above using other samples.RESULTS AND DISCUSSIONIn this experiment, the determination of the molar masses of volatile liquids namely acetone, ethyl acetate and ethyl alcohol. The determination of molar mass in Dumas method experiment uses the ideal gas law: knowing the pressure, volume, and temperature of a gas sample allows us to know the number of moles and knowing the mass that corresponds to that number of moles allows computation of the molar mass. Regarding to the percentage difference of each sample, it obtain 17.21%, 9.13% and 6.93% respectively. There are many factors which can be accounted for these percentage errors. One of which is the intermolecular forces of each compound. Ethyl alcohol for instance has the ability to form hydrogen bonds due to the presence of lone pairs of electrons in the oxygen atom behaving non-ideally. Knowing that an intermolecular forces for gases, deviates from behaving ideally at conditions with high pressures and low temperatures. The molecular volume and the intermolecular forces are some of the variables which create deviations from the ideal behavior. And generally speaking, the larger and stronger these are, the greater the error in the determination. Increasing the amount of the liquid sample injected in the flask doesnt matter for when all the liquid vaporizes it will contain constant maximum amount of vapor and the excess exits through the tiny orifice, this happens at equilibrium Sample 1Sample 2Sample 3

Sample nameAcetoneEthyl AcetateEthyl Alcohol

Trial 1Trial 2Trial 1Trial 2Trial 1Trial 2

Mass of vapor, m0.6470.3190.5530.4530.3540.32

Moles of vapor at T, V and P, n5.1297 x 10-35.0296 x 10-34.9265 x 10-34.8573 x 10-34.9145 x 10-34.9427 x 10-3

Estimated molar mass of vapor, m/n126.12763.42112.2593.2672.031764.7419

Moles of air displaced by the vapor at TR5.835 x 10-45.7385 x 10-45.8027 x 10-45.754 x 10-45.788 x 10-45.7886 x 10-4

Molar mass of air, calculated28.8428.8428.8428.8428.8428.84

Mass air displaced by the vaporized liquid0.016820.016540.01670.016590.016690.016694

True mass of vapor that occupied the flask at the boiling temperature of water0.663920.333540.56970.469690.370610.336694

Corrected molar mass of the vapor (average)68.06996.155549.2736

Molar mass (literature value)58.07888.10446.08

% difference17.21%9.13%6.93%

Table 1. Determination of Molar Mass of a Volatile Liquid by Vapor-Density Methodwhen the pressure inside equals the pressure outside. This is evident in the close quantitative relation of the data have gathered (especially that for ethyl alcohol) for the mass of the vapor. Another important point to be considered is that the flask used before the liquid is added or before the liquid is formed contains a fixed amount of air. This air has mass and contributes to the overall mass of the flask. The volatile liquid sample when boils, it exerts a vapor pressure in which when the liquid was placed in the vessel, some of the air is pushed out of the flask because of the vapor pressure exerted by the liquid. Therefore, when the container is massed again, there is air that is missing and this should be considered in the final mass if the final mass of the air is to be determined by difference. This is visible with the results showing that the calculated molar mass of the vapor for the three samples are all less than their literature value. Adding the missing mass of air at room temperature and pressure brings closer to the true value.The experiment is susceptible; thus the following measures are to be taken into consideration in order to lessen and to avoid possible sources of error:

Improper cutting of the aluminum foil which serves as the cap. Excess foil should be removed for it may serve as a hiding place for water vapor while boiling and thus may add up to the total weight of the flask plus the condensed vapor.

Improper injecting of the syringe in the foil which may create a bigger hole and thus cause the volatile liquid to rapidly escape from the flask.

Incompletely remove the water from the outside of the flask after allowing it to cool down. Condensed water under the edges of the cap can change the mass of the container significantly.CONCLUSION AND RECOMMENDATIONThrough the experiment, it is concluded that among the three samples given, the ethyl acetate has the one with the highest value for molar mass of the vapor which is 95.634 percent . Probably, one of the main reasons for this is that among the three, ethyl acetate has the largest molar mass. Second to the highest is the acetone that has 72.4256 percent and lastly is the ethanol which has found to have 52.4278 percent.

With these result, it is found to have a 16.21 percent error for acetone, 9.13 percent error for ethyl acetate and 6.93 percent error for ethanol. Probable reason for this is perhaps human error. It is the way on how it is the sample injected in the Erlenmeyer flask because some of it after injected left a little bit big hole in the aluminum foil. It is strongly recommended to use smaller needle to lessen percentage errors.

REFERENCES1. Experimental Studies for General Chemistry, Malcolm F. Nicol, Arlene A. Russell, Eleanor D. Siebert 2. Advanced Chemistry with Vernier & Laboratory Experiments for Advanced Placement Chemistry, Sally Ann Vonderbrink, Ph. D.3. Laboratory Manual for Chemistry, Lawrence Epstein4. Physical Chemistry Laboratory Manual, Part 1 (2006), Alvin R. Caparanga, John Ysrael G. Baluyut and Allan N. Soriano

5. Elementary Principles of Chemical Processes, 3rd ed., Felder, R.M. and Rousseau, R.W. 2000.6. Chemistry Principles and Reactions, 4th ed., Masterton, W. and Hurley, C. 2001APPENDICES

Appendix 1: Useful Mathematical MethodsGeneral strategies

If the vapor in the flask is assumed to be an ideal gas, equation (1) can be used to determine its molecular weight. In equation (1): MW =mRT/(PV)

The temperature of the vapor is that of the boiling water bath.The pressure of the vapor is that of the atmosphere.The volume of the vapor is that of the flask and glass tube.The mass of the vapor is determined by removing the flask from the waterbath, allowing it to cool, weighing the flask and condensed liquid, and subtracting the mass of the empty flask.

Pressure Unit Conversion Constants

Conversion FactorsTo convert Into Multiply by

atmosphere bar 1.01295

atmosphere dynes/cm2 1.01295 x 106

atmosphere in. Hg 29.9213

atmosphere in. water 406.86

atmosphere kg/cm2 1.03325

atmosphere mbar 1012.95

atmosphere mtorr or micron Hg 7.6 x 105

atmosphere Pa or N/m2 1.01295 x 105

atmosphere PSI or lb/in2 14.696

atmosphere torr or mm Hg 760

bar atmosphere 0.9872

bar dynes/cm2 1 x 106

bar in. Hg 29.54

bar in. water 401.65

bar kg/cm2 1.02

bar mbar 1000

bar mtorr or micron Hg 7.5028 x 105

bar Pa or N/m2 1 x 105

bar psi or lb/in2 14.503861

bar torr or mm Hg 750.2838

torr or mm Hg bar 1.3328 x 10-3

torr or mm Hg dynes/cm2 1.3328 x 103

torr or mm Hg kg/cm2 1.3595 x 10-3

torr or mm Hg in. Hg 3.937 x 10-2

torr or mm Hg in. water 0.5353

torr or mm Hg mbar 1.3328

torr or mm Hg mtorr or micron Hg 1000

torr or mm Hg Pa or N/m2 133.28

torr or mm Hg psi or lb/in2 1.934 x 10-2

Appendix 2: Miscellaneous Useful InformationGases and liquids with relatively large intermolecular forces and large molecular volume do not calculate according to the ideal gas laws equation; in fact some compounds that we normally consider as liquids deviate significally from ideal gas behaviour at temperatures at or slightly above their boiling points. Under these conditions, van der Waals equation, a modification of the ideal gas law equation, can be used to correct for the intermolecular forces and molecular volumes in determining the moles of gas present in the system:

(P + n2a / V2)(V- nb) = nRTIn this equation, P,V,T,R, and n have the same meanings

a, is an a experimental value that is representative of the intermolecular forces of the vapour, and b is an experimental value that is representative of the volume of the molecules.

If a more accurate determination of the moles of vapour, nvapor, in the flask is required, van der Waals equation can be used instead of the ideal gas law equation. Some values of a and b for a number of low boiling point liquids are listed in table.

van der Waal's Constants for Real GasesThe van der Waal's equation of state for a real gas is:(P + n2a / V2)(V- nb) = nRTTo convert 'a' into atm L2/mol2 multiply by 0.986 atm/bar

To convert 'a' into kPa L2/mol2 multiply by 100.0 kPa/bar

Molecular FormulaNameab

C2H5OHEthanol12.560.08710

C3H6OAcetone16.020.1124

C4H8O2Ethyl acetate20.570.1401

Experiment 01 Group No. 4 08 June 2010

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