Raoult's law and vapor pressure measurement

(1) The mercury barometers (illustrated) are con- structed from 110 mm long, 8 mm i.d. Pyrex tubes, flared at the bottom and with 3-mm vacuum stopcock...
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Remarks This demonstration offers a dramatic display of the operation and effectiveness of ion exchange columns and illustrates precipitation and complexation reactions in relation to hard water. Hard water is softened by five processes: distillation (performed prior to the demonstration), precipitation of Ca2+ (as CaCOa), complexation of Ca2+ (as CaEDTA2-), ion exchange (exchanging 2Na+ for Ca2+), and deionization. The Dresence of Ca2+ in hard tap water and its objectionable-property of. forming preipitates (mainlv calcium stearate) with soaD is shown. The presence bf ion; species in certain water samples is indicated by a conductivity device.

Raoult's Law and Vapor Pressure Measurement Submitted by:

Checked by:

Ned Egan a n d P e t e r C. Ford University of California Santa Barbara, 93106 A. R. Burkett Dillard University New Orleans, Louisiana 70122

Preparation (1) The mercury barometers (illustrated) are constructed from 110 mm long, 8 mm i.d. Pyrex tubes, flared at the bottom and with 3-mm vacuum stopcocks sealed to the top. These are wired onto a rigid hoard in parallel to a meter stick. The flared ends are submerged vertically into a pool of mercury in a beaker. Use a vacuum pump to evacuate the tubes uia the stopcocks, then close the stopcocks and adjust the height so the bottom of the meter stick is a t the Hg pool surface. The height of the Hg column should "


(2) Prepare stoppered flasks

df pure

ethyl ether and of 50 mole % etberltoluene. Also needed are two hook-shaped eyedroppers (bent Pasteur pipets can be used). Demonstration (1) Read the height of the Hg column then inject (using a pipet) -0.5 ml ether into the lower end of one barometer so that it can rise to the top of the column. Read the new height. The difference of the initial and final heights is the vapor pressure of ether in equilihrium with its pure liquid (-400 mm (Hg) at 2OoC). (2) Use Raoult's Law to calculate the vapor pressure of ether in equilihrium with a 50% mole solution of ether and the relatively non-volatile solute toluene. Introduce -0.5

ml of this solution into the second barometer. Allow about 30 sec for the system to come to equilibrium then measure the new height of the barometer. Compare the measured vapor pressuie to the calculated value. Remarks Upon introducine a volatile liauid (ether) into a barorneter sipporting a H; column equal to the atmospheric pressure, the height of that column quickly drops an amount approximately equal to the vapor pressure of the liquid. Introducing an ether solution of a non-volatile solute into an identicibarometer causes a smaller drop in the Hg column which can be compared to the vapor pressure calculated ac= P,~,.,t(l - x.,I,~)~. cording to Raoult's Law, [Pa,~.tian Agreement of measured and calculated values is within 10%. This demonstration offers a simple and dynamic method to reinforce the student's understanding of the term "vapor pressure in mm Hg" by direct measurement of the height of a mercury column to show how vapor pressure can be measured, and to show how Raoult's Law can apply to a real system.

Volume 53, Number 5, May 1976 / 303