COLLIGATIVE PROPERTIES

Transcription

1 COLLIGATIVE PROPERTIES Purpose The purpose o this experiment is to investigate colligative properties o solutions and how they can e used to determine molecular weight o solute. Discussion From the Clausius-Clapeyron equation, P ln P H T ( T ) = () RT T the vapor pressure o a solution o a non-volatile solute may e compared to that o the pure solvent. It can e shown that the oiling point elevation is RT0 T = m () H V where T 0 is the oiling point o the solvent, is molecular weight o solvent, HV is heat o vaporization o solvent, and m is the molal concentration. This equation is valid or ideal solutions and or small temperature changes. The equation may e rewritten as T = m (3) where is the molal oiling point constant. Since the molal concentration is W m W where, W = mass o solute in grams, = molecular weight o the solute, W = mass o solvent in grams = (4)

2 Comining oth equations the molecular weight o the solute is W T W = (5) Analogous equations may e derived or reezing point depression. T = m (6) W = (7) T W Values o and are est determined experimentally, ut may e determined rom equation, as presented in the ollowing tale: Solvent B.P..P. ( C) ( C / molal) ( C) Water Benzene Caron Tetrachloride Camphor Cyclohexane Equipment and Chemicals ( C / molal) Boiling Point: Cottrell oiling point apparatus, Beckman thermometer, micro-urner, solvent (CC4), naphthalene, other solute ( unknown, diphenyl, enzoic acid, salicylic acid, p-nitrotoluene. elting Point: Beckman reezing point apparatus, Beckman thermometer, cyclohexane, naphthalene, other solute ( unknown, such as p-dichloroenzene, iphenyl, p- romochloroenzene).

3 Directions Boiling Point:. Pipet 75 ml CCl 4, with a ul, into the apparatus.. Insert Beckman thermometer. Note: this is a special thermometer with a six degree range. Each division is 0.0. The mercury in the reservoir may e varied to coincide with the desired temperature. Thus, asolute temperatures are not measured, only temperature changes. A hand lens may e used to acilitate reading the thermometer. 3..Boil solvent gently and determine oiling point. The thermometer ul must e in contact with the oiling solution, not the condensing vapor which is pure solvent. Take several readings, which should agree with ± Add an amount o naphthalene which will give a - temperature change. easure the oiling point as eore. (Use pellet press to acilitate sample handling.) 5. Add another quantity o naphthalene and again determine the oiling point. 6. From the data determine two values o and the average equation 5.) 7. Pour solution into waste solvent ottle, not the drain. 8. Repeat steps - 5 with an unknown instead o naphthalene. 9. Determine average molecular weight o unknown. Use equation 5.. (Use a rearranged

4 Directions elting Point. Pipet 0 ml (to nearest 0.0 ml) cyclohexane into apparatus.. Place tue in larger test tue (air jacket) which is sumerged in ice ath. Freezing point o cyclohexane is measured with Beckmann thermometer to nearest 0.00 on a cooling cycle. elt and repeat. 3. Dissolve aout 0.03 g (measured to ± g) naphthalene. To determine melting point, the solution is rozen to a slurry, the test tue is placed in the air jacket, and the solution is allowed to warm to several degrees aove the melting point. Record the temperature at 30 second intervals. Plot temperature vs. time. The melting point is the intersection o the two straight-line portions o the curve. Repeat until melting points dier y no more than 5%. 4. Add another quantity o naphthalene and again determine melting point. 5. From the data determine. 6. Pour solution into waste solvent ottle, not the sink. 7. Repeat steps - 4 with an unknown instead o naphthalene. 8. Determine molecular weight o unknown.

5 olecular Weight Apparatus-Boiling Point, Cottrell-Choppin, T. Grindings, PYREX Brand Glass. Basic Cottrell design, ut o heavy construction with separate pump, reducing ragility and a three way stopcock permitting the withdrawal o samples rom oiling tue or condenser or the return o condensate to the vessel. For technique, reer to Journal o Chemical Education, 4, 49 (947). For thermometer with T grinding, see S For condenser, see S-530-B. T grindings, outer part, No. 9/38 are provided to receive thermometer and condenser. The inner and outer elements are joined y a T grinding No. 45/50. Overall height, 30 mm: overall width, 5 mm. olecular Weight Apparatus- Freezing Point, Beckmann Improved orm, intended or use o sample pellets prepared in presses. Consists o a reezing point tue, 90 mm y 5 mm, with a side tuulature or the introduction o pellets, tu eing supported y a ruer stopper in an air jacket tue 50 x 37 mm which rests in a corresponding hole in a nickel plated rass cover itted to a glass reezing jo, S D. The reezing point tue is equipped with stopper through which are inserted a Beckmann thermometer and a chromel wire stirrer, the latter operating through a small piece o glass tuing inserted in the stopper. The internal stirrer is inished in a ring orm agitating end which moves vertically within the reezing point tue and is so adjusted as to clear the Beckmann thermometer ul when located in the center o the tue.

6 Thermometer-Beckmann Dierential, Upward Scale For accurate measurements o very small temperature dierentials such as those encountered in oiling point determinations and calorimeter measurements. The range covers approximately 5 C graduated in /00 sudivisions. Has a mercury reservoir with auxiliary scale providing a rough indication o the amount o mercury to e trapped in the reservoir to provide the desired operating range. A dropping trap is included. This instrument can e used only or determining temperature dierences. Thermometer-Beckmann Dierential, Downward Scale Similar to the Thermometer, ut or measurement o small dierentials in reezing point and molecular weight determinations, and with scale reading downward. Sudivided to /00.