ISOPIESTIC METHOD .for the DETERMINATION of MOLECULAR WEIGHTS CHARLES M. MASON AND HAMILTON M. GARDNER University of New Hamphire, Durham, New Hampshire
A simple ap@ratus for the measurement of the relative vapor pressure of aqueous solutions is described. An ecperiment for the determination of molecular weights APPARATUS from the vapor-pressure lowering is outlined. This This apparatus is an adaptation of that described by experiment is shown to be suitable for students i n the Smclair (2) and used by Robinson and Sinclair (3) in elementary physical chemistry laboratory.
their measurement of the activity coefficients of salt solutions. The apparatus consists essentially of a brass vacuum XCEPT for the none too satisfactory apparatus of Menzies (I) there seems to be no practical desiccator as shown in Figures 1and 2. The solutions under investigation are weighed into method for the determination of molecular 30-cc. nickel crucibles. These are placed in copper weights from vapor-pressure lowering in the elementary rings soldered to the bottom of the desiccator. This physical chemistry laboratory. I n the present experiment the vapor pressure of a is to prevent them from tipping over. The rings are solution is measured in terms of the vapor pressure of a punched with holes to allow free circulation of the standard solution placed in the bottom of the apparastandard solution. Samples of the solution are placed in a closed con- tus. The desiccator is connected through a brass tainer with samples of the standard solution. When T-tube in the cover to an open-tube mercury manometer not showninthe diagram. equilibrium is attained the vapor pressure of the soluTop View Cover Removed The apparatus is evacution will be the same as that of the standard. The ated through the T-tube molecular weight is then calculated from the vaporand a shut-off (xlass pressure lowering as determined from the vapor presstopcock) by meaniof a sure of the unknown solution. water aspirator. THEORY INVOLVED The apparatus is completely immersed in a It is a well-known fact that if the vapor pressure of a water thermostat to presolution could be determined, the molecular weight of vent condensation of solthe dissolved substance could easily be calculated. vent on the cover if the From Raoult's Law we have : thermostat is above room temperature. The aDis suspended in lo'& such a manner that i t may be rocked or oscilwhere x = mol fraction of solute, anonefip lated through a 15' arc about 6fty times a ming = weight of solute present, G = weight of solvent present, ute. ~ h *was accomm = molecular weight of the dissolved solute, plished by swinging the M = molecular weight of the solvent, apparatus in a frame on po = vapor pressure of pure solvent, the round supports inp = vapor pressure of solvent in the solution. dicated in Figure 1. A We may then write: discarded windshield wiper served very well to PogM M m=(2) operate the apparatus. (90 - 9)G - -6 It was found that a for the molecular weight of the solute. If we use dilute metal apparatus permitsolutions we may assume the number of moles of solute Si& L 4 w -7nru Csnter ted the attaining of equito be negligible compared to the solvent and write for E,F, I,-V~oB-PRESSmE AP- l i b r i u m much more PARATUS quickly than a glass one. the molecular weight: 188
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The whole procedure is based on the use of about 2 cc. of solution in the crucibles. Enough potassium chloride (0.0746 g.) is weighed into each of two tared nickel crucibles to make approximately 0.5 molal solution with 2 cc. of water. Enough of the unknown (calculated) is then weighed into each of the other two tared nickel crucibles to give about the same lowering of the vapor pressure as the potassium chloride solutions. Two cc. of water is then added to each crucible and they are placed in the apparatus. A depth of about 1 cm. of 0.5 molal potassium chloride is placed in the bottom of the apparatus to increase the heat transfer and speed up the equilibrium. The apparatus is then evacuated to about 30 mm. of mercury and the shut-off to the water aspirator is closed. The whole is then placed in the bath and rocked for from 4 to 7 days. A slight rise in pressure on the manometer does no harm as this will not change the partial pressure of water inside the apparatus. If the pressure should rise above 100 mm., equilibrium will not be obtamed in the time allowed. At the end of the time specified the apparatus is removed and carefully opened. The crucibles are placed covered in a desiccator with 0.5 molal potassium chloride in the bottom. This prevents evaporation. They are wiped off with a towel and weighed as rapidly as possible. From the tared weight of the crucible and the salt the weight of water present is easily obtained. The molality of the solution is then calculated from these data. The vapor pressure of a potassium chloride solution of this concentra-
EXPERIMENTAL DETAILS
The procedure consists in bringing to the same vapor pressure a standard solution of known vapor pressure and the unknown. For the standard solution we have chosen potassium chloride. The vapor pressures of aqueous solutions of this substance have been carefully determined over a wide range of concentration. Data for these vapor pressures, calculated from recent experimental data (3), are given in Table 1 and shown as plotted for use in the calculations in Figure 3. TABLE 1
0.0 0.2 0.4 0.6 0.8
1.0 1.2 1.4 1.6 1.8 2.0
Molality of KCI.
CURE
3.-VAPOR
POTASSIUM CALOSOLUTIONS
PRESSURE OP. RIDE
tion is obtained from Table 1. A curve such as is shown in Figure 3 is very useful for this purpose. This gives the vapor pressure of the unknown solution from which its molecular weight is calculated by equation (2). Table 2 gives a summary of results obtained by students using this apparatus. TABLE 2 MOLBCVLAI( WSIOHTSOBTAINSD BY I S O P ~ ~ TMBIHOD IC Sludcnl Subslancc No. 1 No. 2 Mean Thcadicnl % Ena 1 2
sucrose
3
sucrose
4
rnannital
sucrwe
339.7 337.9 342.9 182.71
341.6 339.3 344.2 186.37
340.6 338.6 343.5 184.5
342.17 342.17 342.17 182.1
0.42 1.04 0.38 1.31
DISCUSSION
The results obtained, when compared with those usually obtained by elementary students with the
cryoscopic and ebullioscopic methods, justify the use of the apparatus described. This apparatus is inexpensive and durable. The time required for performance of the experiment does not exceed two hours. The authors are indebted to Mr. W. J. Daniels for aid and suggestions in the design of the mechanical equipment and to Mr. G. F. Temple for preparing the drawings. LITERATURE CITED
(1) nlrrrzrcs, "A method for determining the molecular weights of dissolved substances by measurement of lowering of w o.w . uressurc." I . A m . Chrm S06..32. 161F21 (1911) (2) SEWLAIR,"A &ple method for ace.i&te determinatirk of vapor pressures of solutions," J. Pkyr. Chem., 37, 495-504 (1933). (3) ROBINSON AND SZNCNCLAIR, "The activity coefficients of the alkali chlorides and of lithium iodide in aqueous solution from vapor pressure measurements," 3. Am. Chem. Soc., 56, 1830-5 (1934).
