EXPERIMENT #4 Freezing Points, Cooling Curves, and Molar Mass Determination

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Nickolas Lyons
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1 OBJECTIVES: EXPERIMENT #4 Freezing Points, Cooling Curves, and Molar Mass Determination Observe temperature versus time and record data for pure acetic acid cooled in an ice-water bath Plot temperature versus time for the above data Determine the freezing point of pure acetic acid Determine the freezing point of acetic acid containing an unknown dissolved material Calculate the molar mass of the dissolved material BACKGROUND: The physical states matter exhibits -- solid, liquid, or gas -- depend on temperature and pressure. Transition of a substance from solid to liquid and from liquid to gas occurs when the temperature is sufficient to supply enough kinetic energy to the individual molecules in order for them to overcome the forces of attraction for each other. For the reverse process, when a pure liquid is cooled, a temperature is eventually reached at which its solid form begins to separate. This temperature, where liquid and solid exist simultaneously, is called the freezing point of the substance. The temperature where a solid changes to a liquid is called the melting point. When a liquid solution is cooled, a temperature is eventually reached at which a solid begins to separate from the solution. This solid is pure solvent. The temperature where a solution and solid exist simultaneously is called the freezing point of the solution. The solid component is the pure solvent. If you have ever observed a container of a soft drink partially freeze, the solid is pure water -- the solvent. Another example is the formation of sea ice. The Arctic and Antarctic seas are very concentrated salt solutions. When the temperature is low enough for the sea to freeze, the solid ice is pure water. The remaining liquid solution is an even more concentrated salt solution. The freezing point of a liquid is lowered by the presence of dissolved material. The solute molecules interfere with the attractive forces between solvent molecules, thus hindering the ability of the solvent to form a solid. In order for the solvent to solidify, an even lower temperature (hence a depression in the freezing point) is needed to overcome the interference of the solute. The freezing point of a pure liquid is obtained from time-temperature observations. The graph of these observations is called the cooling curve for the substance. The time-temperature data for a pure solvent and for a solution are shown in FIGURE I on the following page. The freezing point of the pure liquid is obtained by extrapolating the flat part (plateau) of the solid-liquid line of the cooling curve to the temperature axis. The cooling curve for the solution falls below that of the pure solvent. Note that there is no plateau in the cooling curve of the solution. The freezing point of the solution is obtained by extrapolating the sloping solid-liquid line of the cooling curve to the liquid line of the cooling curve and then across to the temperature axis. The lowering of the freezing point of a solution compared to the pure solvent is a colligative property of the solvent. A colligative property is one depending on the number of particles (molecules or ions) in solution. A type of concentration unit which is related to the number of particles in a solution is called the molality. Molality, m, is defined as the number of moles of solute per g (1.000 kg) of solvent. Notice the differences and similarities between molality and molarity. 33 P a g e

2 EXPERIMENT #4 FREEZING POINTS FIGURE I: Cooling Curves for a Solvent and a Solution FIGURE II: Modified Beckmann Freezing Point Apparatus moles of solute moles of solute molality = m = = 1000g of solvent kg of solvent The freezing point depression, T is proportional to the molality of the solution. The proportionality constant Kfp, called the molal freezing-point depression constant, is a property of the solvent. T =(Kfp)(m) The table below lists several solvents, their freezing points, and freezing point depression constants. TABLE I: Solvent Freezing Points and Molal Freezing-Point Depression Constants Solvent Freezing Point, C Kfp, C/m HC2H3O2 (acetic acid) C6H6 (benzene) CHCl3 (chloroform) H2O (water) C10H8 (naphthalene) C6H12 (cyclohexane) para-dichlorobenzene P a g e

3 EXPERIMENT #4 FREEZING POINTS Example: What is the molar mass of urea if the freezing point of a solution containing 15.0 g of urea in g of naphthalene is 63.5 C? First, find T: T = freezing point of solvent freezing point of solution = 80.6 C C = 17.1 C Second, find molality: Third, find moles: Fourth, find molar mass: T = (Kfp)(m) 17.1 C = (6.9 C/m)(m) 17.1 C = m 6.9 C m 2.48 = m moles of solute m = kg of solvent moles of solute 2.48 = kg = moles mass moles = molar mass mass molar mass = moles 15.0g = 0.248mol = 60.5 g/mol 35 P a g e

4 EXPERIMENT #4 FREEZING POINTS PROCEDURE: 1. Work in pairs. Do not dispose of any material by flushing down the sink. 2. A modified Beckmann apparatus (FIGURE II) will be used to determine the freezing point of solvent and solution. A test tube containing the solvent or solution will be placed in a beaker of ice water. The thermometer will also serve as the stirrer. Remember that the thermometer is fragile, so handle it with care. In this method the solvent or solution is cooled at a slow, steady rate, and time/temperature readings are recorded until a solid has formed. Graphs are then plotted and freezing points are obtained from the graphs as described above. 3. Measure approximately ml of the solvent, acetic acid, into the sample tube (25 mm x 150 mm) and place the apparatus in a water bath. Add ice cubes to the water bath at such a rate that the temperature of the acetic acid (or the acetic acid solution) slowly becomes lower. Constant stirring of the solvent is maintained throughout the cooling as the time/temperature readings are recorded every 20 seconds for the first two minutes and then at one minute intervals until a time/temperature trend line is clearly established. The constant stirring initiates crystallization of the solution and ensures a uniform temperature throughout the liquid. Readings may be taken until complete solidification of the solvent occurs. 4. The sample tube is removed and warmed with warm water to melt the solid acetic acid and bring the temperature of the solvent to about 25 C. Repeat Step 3 or go on to the next step. 5. Obtain another clean, dry sample tube (25 mm x 150 mm). Weigh out g, to the nearest thousandth of a gram, of your unknown on a square of weighing paper. Record the mass on your data sheet. Add the unknown to the dry tube. Place the dry tube containing the unknown solute in a 600 ml beaker and weigh on a balance. Add the ml of thawed acetic acid to the tube and record the mass. The difference gives the mass of the acetic acid in the solution. Record this mass on your data sheet. 6. The sample tube containing the solution is placed in a cooling bath and the freezing point of the solution is determined. Time/temperature readings are made until complete solidification occurs. 7. On the same piece of graph paper plot temperature (vertical axis) versus time (horizontal axis) for pure solvent and for solution. Obtain T from these graphs. 8. Melt the solution in a warm water bath and repeat the cooling process to collect a second set of data. WASTE DISPOSAL Rinse the contents of the test tube into the waste beaker in the hood using acetone as the solvent. The test tube can now be safely washed in the sink. 36 P a g e

5 NAME Section Date DATA AND CALCULATIONS: Freezing Points TABLE II: Freezing Point of Pure Solvent Freezing point of acetic acid (from graph) TABLE III: Freezing Point of Solution Trial I Trial II Unknown Letter Mass of unknown (solute) Mass of test tube, beaker, unknown, and acetic acid Mass of test tube, beaker, and unknown Mass of acetic acid, g Mass of acetic acid, kg Freezing point of solution (from graph) T of solution Kfp of acetic acid (from TABLE I) Molar Mass of Solute Clearly show molar mass calculations. Set up equations, rearrange for unknown, substitute numbers, then calculate values. Use the reverse side of this page if necessary. 37 P a g e

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7 NAME Section Date Time/Temperature Observations Solvent Solution Time (min:sec) C Time (min:sec) C 0:0 0:0 0:20 0:20 0:40 0:40 1:00 1:00 1:20 1:20 1:40 1:40 2:00 2:00 2:20 2:20 2:40 2:40 3:00 3:00 3:20 3:20 3:40 3:40 4:00 4:00 4:20 4:20 4:40 4:40 5:00 5:00 5:20 5:20 5:40 5:40 6:00 6:00 7:00 7:00 8:00 8:00 9:00 9:00 10:00 10:00 11:00 11:00 12:00 12:00 39 P a g e

8 Time/Temperature Observations Solvent Solution Time (min:sec) C Time (min:sec) C 0:0 0:0 0:20 0:20 0:40 0:40 1:00 1:00 1:20 1:20 1:40 1:40 2:00 2:00 2:20 2:20 2:40 2:40 3:00 3:00 3:20 3:20 3:40 3:40 4:00 4:00 4:20 4:20 4:40 4:40 5:00 5:00 5:20 5:20 5:40 5:40 6:00 6:00 7:00 7:00 8:00 8:00 9:00 9:00 10:00 10:00 11:00 11:00 12:00 12:00 40 P a g e

9 NAME Section Date ADDITIONAL ASSIGNMENT I: Freezing Points Use freezing point and Kfp data in TABLE I on page What is the molality of a solution that contains 3.00 g of guanidine (molar mass = g/mol) in g of benzene? 2. How many grams of NaNO3 would you add to g of water in order to prepare a solution that is molal in sodium nitrate? 3. What would be the freezing point of a solution containing 19.5 g of biphenyl (C12H10), the solute, dissolved in g of naphthalene, the solvent? 41 P a g e

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11 NAME Section Date ADDITIONAL ASSIGNMENT II: Freezing Points Use freezing point and Kfp data in TABLE I on page Calculate the freezing point of a solution containing 13.0 g of benzene (solute) in g of chloroform(solvent). 2. A solution containing 2.00 g of an unknown substance in 25.0 g of naphthalene was found to freeze at 75.4 C. What is the molar mass of the unknown substance? 43 P a g e

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Experiment #4. Molar Mass by Freezing Point Depression

Experiment #4. Molar Mass by Freezing Point Depression Introduction When a nonvolatile solute is dissolved in a solvent, the freezing point of the solution is lowered. This process is called Freezing Point