Solubility of C02 A Variation of the Experiment Measuring Standard Molar Volume of a Gas David F. Koster and Russell F. Trimble Southern Illinois University at Carbondale, Carbondale, IL 62901 The measurement of the molar volume of a gas is a classic experiment in the general chemistry laboratory Q p i cally, dihydrojien ( H ~isI generated by the action of'acid on a weighed amount of magnesium or zinc and mllected over water. The pmcedure and calculations need no further description. Even when run in duplicate or triplicate, the exueriment is short for a three-hour lab ueriod. and students knd the repetition boring. We offer the foliowing experiment. a variant of Dudek's carbonate uroiect ( I ) . in which C02is formed in a "molar volume" experiment and the difference between the expected and actual volume of C02 used to obtain the solubility of the gas. The result, perhaps because of compensating errors, agrees satisfactorily with the literature (2,3). In the first part of the experiment, the molar volume of hydrogen is determined in the usual way1 The student then weighs a small marble chip2 (-0.10-0.15 g) and places i t on the bottom of a beaker of water used as the collection trough. To a closed 25-mL buret, the student adds 5 mL of 6M HCI and then fills the buret to the top with water, closes the open end with a finger, and places the buret upside down in the beaker directly over the chip. The HCI diffuses downward and reacts with the calcium carbonate to form C02, which rises in the buret. The student notices that, although C02bubbles are rising into the buret, water is not immediately displaced fmm the buret; only after the water in the buret has been saturated with COI does a gaslwater interface appear. When the reaction is over; i.e., when all of the CaCO, has reacted, the "buret readings" of the water level inside and outside the buret3 are recorded, along with the barometric pressure and water temperature. In addition to the correction for the uressure of water vapor, the C02pressure must be corrected for the inequality of the water levels. This is done by measuring (in millimeters) the difference between the water levels inside and outside the buret. and dividine bv 13.6 to obtain the uressure correction in mm Hg to iubtracted or added, de'In oJr partlcLlar case, the ack of e-o~ometendlctates that we use
mverteo, close0 oJrets nstead As a prel mmary proceoure we have tne StJoenfsf I the bLret to tne top w th water that 1s tnen dra ned Inlo a weighed beaker until the level is at the O-mL mark. From the weight and density01the water, the volume of the uncaiibrated top sectionof the buret can be calculated. Water is then brought to the last calibration (25mL in our case), and the water from that line to the stopcock is collected and weighed in order to determine the volume of the uncalibrated bottom section. 'M123-3 in the Fisher 91/92Cataiog. Alternatively, powdered CaC03 can be put in a vial, which is then filled with water, placed in the beaker of water, and the buret lowered over the vial. 31t usually is impossible to bring the interior and exterior water surfaces to the same level. 'Students can use either the molar volume they just measured for hydrogen. or the usual 22.4Umol. 5From the literature (4,the solubility of C02in acid depends on the particular acid; e.g., at25 "C, a =0.756in water, 0.732in 1 M HCI and 0.781 in I M HNO,. or comparison with literature values, the student results must be corrected for any differencein the C02 pressure from 1 atm. 528
Journal of Chemical Education
pending on whether the interior level is above or below the exterior level. The volume of C02 collected is reduced to STP and compared to the volume expected from the mass of carbonate reacted.4 The difference is the volume of gas dissolved (at the C02 pressure in the buret). The volume of water in which the pas dissolved bas to be aw pmximated. Because thegm rirsfwm the bottom, andthe wa& in hsplaad fmm the bunt From the bomm, the 61% ponion of water displaaul h a s been exposed to rand we assume saturated with1CO2.Wuterpaseshmtheburet intorhemst ofthelrakrr, and the gas bubaes come in contact with only the water in the buret. We assume, therefore, that the volume ofwater which the Con saturates is the total volume of the buret. The solvent.~.of m m , IS not pun! water, but a solution of HC'I (in our uw,the total b w t volume is about 30 mL,so the 5 ml.of6 M HCI gives a solution of about 1 MI.The solubility of CO, in 1 M HCI 2, is 965 of that ul pun? water at 15 and 97%at 25 "C (3,.We assume a value of 97%* the temperature range of interest.5~ecausethe solubility ewve is fairly steep in the 2WO "C range, and because there can be several degrees Werence between the water temperature in the same lab section, the instructor will need a table of solubilities, or a plot of them, when gradmg student experiments." In the region of interest, the solubility (3)follows the expression a = 2.350- 0.4939 MT), where a is the volume of gas (reduced to STP)that dissolvesin 1mlof water at 1atm pressure of the gas, and T is the temperature in degrees Celsius. Results from a series of lab classes are summarized in the table. There were 38 cases where two runs were made bv the same student a t the same temuerature. The averaee difference between the two values bf a was 0.01 with-a standard deviation of 0.01. ~
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Literature Cited 1.Dudek, E.P.J. CAsm.Edue., 1881,68,949-959 2. Internotianal Criliml lbbles, Washbum, E. A. Ed.; McGraw-Hill: New York, 1928; Vol 3, p 279. 3Long~BXondbookofchemid?, 13thed.:Dean,J.A.Ed.;13th ed.,M&raw-Hill:New
York 1985:p 10-4.
Students Determination of Solubility of COP at Various Temperatures NO. 2 1
17 12 18 21 22 25 8 6 2 2 2
ail s.d. 0.72k0.17 0.70 0.662f 0.102 0.713 f 0.0807 0.658f 0.090 0.715+ 0.0788 0.686+ 0.172 0.732+ 0.103 0.730k 0.209 0.625f 0.220 0.775f 0.120 0.750f 0.014 0.83and 0.36a
Value from (3)
