The rn a s of the vapor, mvopo , is detem1ined from the mas diffe rence between the "empty" vessel and the vapor-filled vessel.

nlyopor = l71 f1o,k -I- vapor - t11 naSk (18.2)

The molar mass of the compound, Meompound' i then calculated from the acquired data.

111 ,apt.>r

Mcompound = -1'1--. (18.3) VllpOi

Ga e and liquids ",ill relatively large intennolecular forces and large molecular vol umes do lIot b have a cording to the ideal ga taw quarion: i.n fact, some com­pound. that we normally consider (Ii> liquids, such as H20, dev iate significantly from ideal ga behavior in the vapor tate. Under these cond itions, van del' Waals' equation . a mod ification of the ideal gas law equation, can be used to correct for the intennolecu­lar forces and molecular volumes in determining the moles of gas present in the sys­tem:

112a )(p + V2 (V - nb) = nRT (18.4)

In Ihis quation. P, V, T, R. and 11 have the sam meanings as in Equation 18.1. a is an experimental value that is representalive of the intermolecul ar forces of the vapor, and b is an experimental value that is repre enrative of the volume ( r size) of the molecules .

1J a more accurate determination of the mole of vap r, nv.por in the fl ask is re­quir d, van der WaaJ . e ualion can he u ed instead of the ideal gas law equati n. Some values of a anJ h for a number of low-boiling-point Ii juids are Ii ted in Table 18.1. Oth­e may be found in your textbook.

Table 18.1 Van der W aals' Constants for Some Low-Bailing-Point Compounds

a b

Name (U.a~l)mol­

(Limol) Boili ng Point (0C)

methallol 9.523 ().007m 05.0 ethanol 12 .02 0.08407 7X. 5 aCe.lone 13.9 1 0.0994 50.5 propanol 14.92 111 0 19 82.4 h~xanc A .39 0.1735 69.0 cyclohcxane 22.8 1 0. 1424 80.7

pentane 19. I 0. t460 36.0 waleI' 5.46 0.0305 1000

EXPERIMENTAL

PROCEDURE

Procedure Overview: A boilin g water bath of measured temperature is u. ed to a­porize an unknown liquid i.nlo a flask. The volume of the flask is measured by filling the fla:k with water. ' the fl ask L open to the atmosphere, you wi Ll record a baromet­ric pre ure.

You are to complete three [r ials in determin ing the m lcu' mass of your low-boil­ing-pojntliquid. In itially obtai n 15 to 20 mL of liquid from your in true tor. The same apparatus i used for each trial.

A. Preparing the ample L. Prepare the Fl3.! k for tbe Sample. Clea.n a 12S-mL Erlenmeyer fl ask and dry it either in a drying oven or by aUowing it to 'Lir-dry . Do not wipe it dry or heat it

2 over a direct name. Cover tbe dry fla k with a small piee of aluminum foil (Fig­ure 18. 1 and secure it it! a rubber band. Determine the mass (:to.OOI g) of the ~. dr ' flask, alwninum foi l. nd rubber b'U1d.

220 Molar Moss of Volati le liquid

125- m L Er lenmeyer

flask

luminumPin- hole ill foil covers the stopper nd extend

over outside 01 flask

Figure 18.1 Preparation of a flask for the placement of the volatile liquid.

2. Place tbe Sample in the Flask. T ransfer about 5 mL of the unknown LiquiJ jaLO the flask ; again cover the li ask with the aluminum foil and secure the foil WiOl a rubber balld . You do not need to conduct a mass measurement. With a pin. pierce the aluminum foil several times.

3. Prepare a Boiling Water Bath. Half-fill a 600-IllL beaker wiLb water; support it on a wire gauze and. near the lOp, with a supporl ring. Add one or two boiling chips Lo the water. The heat source may be a hot plate or a Bunsen flame--consult with your instructor.

~ ,10° C G.aSl; or dlgita! Ihermometer

l3c

Boiling ch ip: 0 piece of porous ceramic fh ert releases air when heated (the hubbies formed prevent water from becoming w perheated)

Figure 18.2 Apparatus for determining the molar mass of a volatile liquid

I. Place the Flask/Sample in the Batb. Lower the flask/sample into the bath and se­cure it wiLh a util ity clamp. Be certain that neither the fl ask nor the clamp Louches the beaker wal l. Adjust the water level high on the neck of the flask (Figure 18.2).1

2. Heat the Sample to the Temperature of Boiling Water. Genlly heat the waler un til it reaches a gentle boi l. (Caution: Most unknowns are jlamnwhle; use a moderate flame for heating.) When the liquid in the flask or the vapors escaping from the holes in the aluminum foil are no longer visible, continue heating for an­other 5 minutes . Read and record the temperature of the boiling water, "using all certain digits (from the labeled calibratjon marks on the thermometer) plus one uncel1ain digit (the last digit wJlich is the best estimate between the calibration m arks)."

B. Collecting the Data

lyou may choose to wrap the upper portion of the fla sk and beaker with alum inum fa il; thi s will maintain the upper portion of the flask not in the boiling water bath at the same temperature as the bo iling water .

Experi ment 18 22 1

3. Measw'e tlle Mass of the Flask/Sample. Remove the flask and allow it to cool to

room temperature. Sometimes the remaining vapor in the flask condenses; that's O.K. Dry the outside of the flask and determine the mass (:to.00 1 g, use the same balance!) of the flask, aluminum foil, rubber band, and the vapor.

4. Do It Again and Again. Repeat the experiment for Trials 2 and 3. You only need to transfer another 5 mL of liquid to the flask (i.e., begin with Part A.2) and repeat Parts B. I-B.3.

Disposal: Dispose of the leftover unknown liquid In the "Waste Organics" container.

C. Determine the Volum e I. Measure the Volume of the Flask. F ill the empty 125-mL Erlenmeyer flask to and Pres ure of the Vapor the brim with water. Measure the volume (± O. l mL) of the flask by transferring

the water to a 0- or 100-mL graduated cylinder. Record the total volume .

2. Record the Pressure of the Vapor in the Flask. Find the barometer in the labo­ratory. Read and record the atmospheric pressure in atmospheres, "using all cer­tain digits (from the label d calibration marks on the barometer) plus one uncertain digit (the last digit which is the best estimate between the calibration mark )."

D. Calculations I. Molar Mass from Data. Calculate the molar mass of your unknown for each of the three trials.

Appendix 8

2. Determine the Standard Deviation. Refer to Appendix B and calculate the stan­dard deviation for the molar mass of your unknown from your three trials.

3. Obtain Group Data. Obta in the values of molar mass for the same unknown from other chemists. Calculate the standard deviation for the molar mass of the un­known.

222 Molar Mass of Volatile Liquid

Experiment 18 Prelaboratory Assignment

Molar Mass of a Volatile Liquid

Dote Lob Sec. Nome Desk No. ____

1. A 125-mL Erlenmeyer flask has a measured volume of 152 mL. A O.l 77-g sample of an unknown vapor occupies the flask at 97.8°C a pressure of 748 tOlT. Assume ideal gas behavior. a. How many moles of vapor are present?

b. What is the molar mass of the vapor?

2. a. If the volume of the flask is mistakenly recorded as 125 mL in Question I, what is the reported molar mass of the vapor?

b. What is the percent error caused by the error in the recording of the volume of the flask?

6~ - - _ "' re.;;."- 100M....,d:c,irre:.;.: nc.p X 10 errol ­

M ac",al

3. The ideal gas law equation (Equation 18.1) is an equation used for analyzing "ideal gases." According to the kinetic molecular theory that defines an ideal gas, no ideal gases exist in nature, only " real" gases. Van der Waals ' equation is an attempt to make corrections to real gases that do not exhibit ideal behavior. Describe the type of gaseous molecules that are most susceptible to non ideal behavior.

Experiment 18 223

4. a. How is the pressure of the vaporized liquid determined in the experiment?

b. How is the volume of the vaporized liquid determined in the experiment?

c. How is the temperature of the vaporized liquid determined in the experiment?

d. How is the mass of the vaporized liquid determined in the experiment?

5. The molar mass of a compound is measured to be 33 .4, 35.2, 34.1, and 33.9 g/mol in four trials. a. W hat is the average molar mass of the compound?

b. Calculate the standard deviation (see Appendix B) for the determination of the molar mass.

224 Molar Mass of Volatile Liquic

Experime nt 18 Report Sheet

Molar Mass of a Volatile Liquid

Date Lab Sec. Name Desk No. ____

Unknown Number ______________ Trial r Trial 2 Trial 3

1. Mass of dry flask, foil, and rubber band (g)

2. Temperature of boiling water (OC, K)

3. Mass of dry flask, foil, rubber band, and vapor (g)

4. Volume of 12S-mL flask (L)

+ + = total volume

S. Atmospheric pressure (torr, atm)

Calculations

1. Amount of vapor, I1vapor (mol)

2. Mass of vapor, mvapor (g)

3. Molar mass of compound (g/mol)

4. Average molar mass (g/mol)

S. Standard deviation of molar mass

*Calculation of Trial 1. Show work here.

Class Data/Group I 2 3 4 5 6

IMolar mass

Sample Unknown No. !

Experiment 18 225

What is the standard deviation of the molar mass of the unknown for the class?

(OptionaJ) Ask your instructor for the name of your unknown liquid. Using van der Waals' equation and the vaJues of a

and b for your compound, repeat the calculation for the moles of vapor, nvapor (show for Trial 1 below), to determine a more accurate molar mass of the compound.

Calculations (van der Waals' equation)

Unknown Number _ _ ____________ Trial r Trial 2 Trial 3

1. Amount of vapor, i1 v"l"" (mol)

2. Mass of vapor, mvapor (g)

3. Molar mass of compound (g/mol)

4. Average molar mass (g/mol)

*Calculation of n ,apul from van der Waals' equation for Trial 1. Show work here.

Laboratory Question

Circle the questions that have been assigned.

I. Part A.I . The mass of the flask is measured when the outside of the fla :;k is wet. However in Part B.3 the oll ts ide of the fl ask is dried before its mass is measured. a. Will the mass of vapor in the flask be reported too high. too low, or unaffec ted? Explain. b. Will the molar mass of vapor in the flask be reported too high, too low, or unaffected? Explain.

2. Part A. l . From the time the mas~, of the flask is first measured in Part A.i until the time it is fi nally measured in Part B.3, it is handled a number of limes with oily fingers . How does this lack of proper technique affect the reported mass of the vapor in the flask? Explain.

3. Part A.2. The aJ umi.num foil is pierced several times with pencil-size holes rather than pin-size holes . . a. How will this oversight in the procedure affect the mass of vapor measured in Pait B.3? 2.xplain. b. Will the reported molar mass of the liquid be reported too high , too low, or unaffected? Explain.

4. Part B.2, The flask i not only completely filled with vapor, but some liquid also remains in the bottom of the flask when it is remov d from the hot water bath and cooled in Part B.3. How will this oversight affect the reported molar mass of the liquid ... too high, too low, or unaffected? Explain.

5. Part B.2. The Hask i only com pletely filled with vapor when it is removed from the hot water bath and cooled in Part B.3. However when t11 fl a k cool some of the vapor condenses in the f1a k. A ' a result of this observation, will the reported molar mas of the liquid be too high, too low, or unaffected? Explain.

6. Part C.2. The pressure reading from the barometer is recorded higher than it ac tuaJly is. How does this affect the re­ported molar mass of the liquid ... too high, too low , or unaffected? Explain.

226 Molar Mass of Volatile Liquid