A Demonstration Experiment on Partial Molar Volumes Trichloroacetic acid (TCA) dissolves readily in water, and the solution is accompsnied by a large increase in volume. The partial molar volume of TCA can be determined by measuring this increase in volume a t constant temperature and pressure. The simple experiment described below permits the student to visualize the concept of partial molar volume as well as to measure it with a fair degree of accuracy. The figure shows the volumetric flask used. (The flask was siliconized (Silicon Clay Adams) to O.l°C. Then 50 ml of reduce errors caused by wetting.) The scale of the flask is calibrated at 25 water is added to the flask whichis then stoppered and placed in the thermostat. Five samples of 3 4 g TCA, weighed to 0.01 g and kept ins. dissicator containing calcium chloride, me used. Successively, each sample is added to the flask, dissolved (shake flask to homogenize the solution), and placed in the thermostat until eauilibrium is reached (about 10 min). The meniscus reading before and after each addition is recorded. Plot the volume V of the solution against the number of moles nl of TCA added. .k T, P, and the number of moles of solvent, n,, were kept constant, the slope of the curve
gives the partial molar volume of TCA. F2isfairly constant up to 2M and calculated_fromthe above plot equals 88.7 ml/mole. Using this value for V3,the partial molar volumes of water VI m e calculated
V = n,V, +mi?>
3,
from which
The data from a typical experiment (see table) gives TCA added (g) 2.56 5.18 7.44 10.23 12.83 15.92 17.84
Number of moles of TCA (nd 0.0156 0.0316 0.0455 0.0626 0.0785 0.0975 0.1091
7,
=
18.0 ml/mole.
Increment of volume (AV) (ml) 1.3 2.7 3.8 5.4 6.8 8.6 9.6
Total volume (ml) 53.9 55.3 55.6 57.2 58.6 60.4 61.4 JUANA. COCH
VALENTIN L~PEZ
270
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lournal of Chemical Edumfion
