Equilibrium or Slow Change?-An Experiment for the General Chemistry Laboratory An important characteristic of an equilibrium which distinguishes it fram a steady state caused by the rate of change being too slow ta observe is that an equilibrium can be approached from either side. The equilibrium between solid potsssium chloride and its saturated solution can conveniently he used to demonstrate this characteristic. The experiment, which can be completed in a two-hour laboratory period, also illustrates one method of preparing a water soluble salt; the data can be used to practice calculating concentration in units of molarity, molality, mole fraction, and weight percentage and t o gain experience in analyzing errors. The students work in pairs, each preparing a saturated solution of potassium chloride, one by dissolving solid potassium chloride and the other bv a metathesis reaction between saturated aoueous ootassium carbonate and concentrated hvdrochloric acid. From the volume andmass of asample of the saturated solution and the mass of potassium chloride obtained by evaporating the sample, the grams of potassium chloride dissolved in 100 cc of water inbothsaturated solutions are calculated and compared with each other. Of a typical section of 23 students, four obtained results which were obviously in error and were discarded. The average value hy solution was 37.3 gllO0 ec water and the average value by precipitation was 33.4 gI100 cc of water a t 2fi°C. Data collected for five semesters, choosing data for temperatures from 24.0 t o 26.O0C, gave an average value by solution of 35.4 g1100 cc water and an average value by precipitation of 35.6 gllO0 ce water, with standard deviations of 4.08 and 2.98, respectively.' Experimental2
is just acid t o litmus. Preparation of o saturated solution by solution. A mixture of 10 g of solid potassium chloride and 25 ml of water is stirred vigorously for as long as it takes partner to make the saturated solution by precipitation. Seoaration ofsaturatedsolution fram solid and onolvsis o f saturated solution. The tem~eraturesof the two solutions
Weast, R. C., "Handbook of Chemistry and Physics," 57th Ed., CRC Press, Cleveland, 1976, p. B-144 gives solubilities for potassium chloride of 34.7 gI100 cc water a t 20°C and 65.7 g1100 cc water a t IOO'C. 2 A copy of the discussion, procedure, and report form furnixhgd the students will he sent on request and receipt of legalsized, stamped, selfkddressed envelope.
Jean B. Umland Jean A. Fefer University of Houston Downtown College Houston, TX 77002
Volume 60
Number 1
January 1983
59
