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ColHgafive Properties:

Freezing-PointDepression and

Molar Mass

EXPERIMENT

18 To become familiar with colligative properties and to use them to determine the molar mass of a substance. ring and ring stand clamp wire gauze thermometer large test tube wire stirrer

Bunsen burner and hose

600-mL beaker

sulfur, "roll" or precipitated; or unknown solid (2 g) naphthalene (50 g)

2-hole robber stopper with slit

towel wide-mouth glass bottle weighing paper WORK IN PAIRS BUT EVALUATE YOUR DATA INDEPENDENTLY.

The major component is called the

solvent, and the minor component is called the solute. Since the sQlution is primarily composed of solvent, physi- cal properties of a solution resemble those of the solvent. Some of these physical properties, called colligative ptvperties, am independent of the uature of the solute and depend only upon the solute concentration. The colligative properties include vapor-pressure lowering, boiling-point elevation, freez- ing-point lowering, and osmotic pressure. The vapor pressure is just the escaping tendency of the solvent molecules. When the vapor pressure of a solvent is equal to atmospheric pressure, the solvent boils. At this tempera- ture the gaseous and liquid states of the solvent are in dynamic equilibrium, and the rate of molecules going from the liquid to the gaseous state is equal to the rate of molecules going from the gaseous state to the liquid state. It has been found experimentally that the dissolution of a nonvolatile solute (one with very low vapor pressure) in a solvent lowers the vapor pressure of the solvent, which in turn raises the boiling point and lowers the freezing point. This is shown graphically by the phase diagram given in Figure 18.1 You are probably familiar with some common uses of these effects: Anti- freeze is used to lower the freezing point and raise the boiling point of coolant (water) in an automobile radiator; and salt is used to melt ice. These effects are expressed quantitatively by the colligative-property law, which states that the freezing point and boiling point of a solution differ Dom those of the pure solvent by amounts that are directly proportional to the molal

OBJECTIVE

APPARATUS

AND CHEMICALS

DISCUSSION

189

19{}Experiment 18 - Colligative Properties: Freezing-Point Depression and Molar Mass

~,Freezing point of solution ~, Freezing point of solvent f.p. lowering

Temperature~ °C ~

b.p. elevation

FIGU RE

18.1

Phase diagram for a solvent and a solution,

concentration of the solute. This relationship is expressed by Equation [1] for the freezing-point lowering and boiling-point elevation: ~T =Km[~] where AT is the freezir~g-point lowering or boiling-point elevation. K is a constant that is specific for each solvent, and m is the molality of the solution (number of moles solute peril solvent). Some representative constants, boiling points, and freezing points are given in Table 18.1. For naphthalene, the solvent used in this experiment, the molal freezing-point depression con- stant (K~p) has a value of 6.9°C/m,

EXAMPLE 18.1

What would be the freezing point {~f a solution containing of biphenyl (CI2H m) dissolved in I of naphthalene if the normal freezing point of naph- thalene is 80.6~C?

18,1 Molal Freezing-Point and Boiling-Point Constants

FreezingBoiling

Solventpoint (°C)Ktp(°C/m)point{°C)

CH3COOH (acetic acid)16.63.90118.1

CbH 6 (benzene)5,zt5.1280.2

CHCI~ (chloroform)-63.54.6861.3

C~HsOH (ethyl alcohol)-114.I--78.4

H~O (water)0,01,86100.0

Ci~ (naphthalene)80.66,9218

C~H~z (cyclohexane)6.620,480,7

Kl}p(°C/ra)

2.93 2.53 3.63 1.22 2.79

SOLUTION:

Laboratory Experiments191

19.5 g0.127 toolMoles C12H10154 g/mo!

Moles C12H10

/0.127 mo!~ naphthalene = ~ ~ ](1~000 g) = 1.27 m

AT= (6.9°C/m)(1.27 m)

= 8.76°C or 8.8°C Since the freezing point is lowered, the observed freezing point of this solution will be

80.6oc - 8.8oc = 71.8oc

Since the molal freezing-point-depression constant is known, it is possible to obtain the molar mass of a solute by measuring the freezing point of a solu- tion and the weight of both the solute and solvent.

EXAMPLE 18.2

What is the molar mass of urea if the freezing point of a solution containing 15 g of urea in of naphthalene is 63.3°C? SOLUTION: The freezing point of pure naphthalene is 80.6°C. Therefore, the freezing-point lowering (AT) is: &T - 80.6°C - 63.3°C - 17.3°C

From Equation [1] above,

17.3°C - Kfpln

17.3°Cm=-

We know that K~p for naphthalene is 6.9°C/m. Therefore, the molality of thissolution is

17.3°Cm = -- - 2.5 m6.9°C/m

Remember that molality is the number of moles of solute per of solvent. In our solution there are urea !n of naphthalene, or of urea in of naphthalene. Thus = 2.5 mol

1 mol = 60 g

The molar mass of urea is, therefore, 60 g/mol

In this experiment you will determine the molar mass of either sulfur or an unknown. You will do this by determining the freezing-point depression of a naphthalene solution having a known concentration of either sulfur or your unknown. The freezing temperature is difficult to ascertain by direct visual observation because of a phenomenon called supercooling and also because solidification of solutions usually occurs over a broad temperature range. Temperature-time graphs, called cooling curves, reveal freezing tem- peratures rather clearly. Therefore, you will study the rate at which liquid

192Experiment !8 ° Colligafive Properties: Freezing-Point Depression and Molar Mass

Liquid

is freezing

Freezing

beginssolution

Solid is cooling

Cooling curve for

pure solvent

Cooling curve for solution

Cooling curves for a solvent and a solution,

naphthalene and its solutions cool and will construct a cooling curve similar to the one shown in Figure 18.2. You will construct cooling curves for both the pure solvent and the solu- tion. Figure 18.2 shows how the freezing point of a solution must be deter- mined by extrapolation of the cooling curve. Extrapolation is necessary because as the solution freezes the solid that is formed is ~ssentially pure solvent and the remaining solution becomes more and more concentrated. Thus its freezing point lowers continuously. Clearly, supercooling produces an ambiguity in the freezing point and should be minimized. Stirring the solution helps to minimize supercooling. PROCEDURE ,j. A. Cooling Curve for Pure Naphthalene Q~ Weigh a large test tube to the nearest 0 01 g. Add about of naphthalene and weigh again. The difference in weight is the weight of naphthalene, Assemble the apparatus as shown in Figure 18.3; be certain to use a split two-hole rubber stopper. Carefully insert the thermometer into the hole that has been slit. Bend the stirrer so that the loop encircles the thermometer. ~ Fill your 600-mL beaker nearly full of water and heat it to about 85°C, Clamp the test tube in the water bath as shown in Figure 18,3. When most o~ the naphthalene has melted, insert the stopper containing the thermometer and stirrer into the test tube; make certain that the thermometer is not rest- ing on the bottom of or touching the sides of the test tube, When all of the naphthalene has melted, stop heating, remove the beaker of water, and dry the outside of the test tube with a cloth towel, Place the test tube in a wide- mouthed bottle that contains a piece of crumpled paper in the bottom to lessen the chance that impact of the test tube with the bottle will cause the bottle to break. The purpose of the wide-mouth bottle is to minimize drafts. (~Record temperature readings every 30 s while you are stirring. When the freezing point is reached, crystals will start to form, and the ~emperature

Wire stirrer

Laboratory Experiments

193

Support thermora~ with split

two-hole rubber stopper Test %ube

600-mL beaker

ater ba~h ~phthaleneintesttube

FIGURE

18.3 Apparatus for determination of cooling /urve.

will remain constant. Shortly after this/the naphthalene will solidify to the

point where you can no longer stir it. t,, "~_.q::~,~ ~, ~,,~ ~- _,~ --~ ~ ~-~Your lab ins[rttctor will direct you to perform either~~,r pro-

cedure C~)

B. Determination of the Molar Mass of

Sulfur

Using weighing paper, weigh to the nearest about 1.2 to ~ of sulfur. CLEAN UP ANY SULFUR SPILLS IN THE BALANCE. Replace the test tube in the water bath and heat until all the naphthalene has melted. Gently remove the stopper, making sure that no naphthalene is lost, and add the sulfur to the test tube. Replace the stopper and stir gdntly until all the sulfur has dissolved. Remove the water bath, dry the test tube with a towel, and insert the test tube in a wide-mouth glass bottle containing a crumpled p~ece of paper. Record the temp, erature every 30 s until all tl~ naphthalene

Cleanup To clean out the test tube at the

end of the experiment, heat the test tube in a water bath until the naphthalene just melts.

Care should be taken

not to heat the thermolneter beyond its teraperature range. Be careful, becattse ~aph- thalene is flammable. Remove the stopper and pour the molten naphthalene on a crumpEed wad of paper. When the naphthalene has solidified, throw both the paper and solid naphthalene into a waste receptacle. DO NOT

POUR LIQUID NAPHTHALENE INTO THE SINK!

194Experiment I8 ¯ Colligative Properties: Freezlng-Point Depression and Molar Mass

C. Determination of the Molar Mass of an Unknown*

Place the test tube in the water bath and heat until all the naphthalene has melted. Using weighing pape~',~weigh about of your unknown to the nearest 0.0I g. Gently remove the stopper from the test tube, making sure that no naphthalene is lost, and add your unknown to the test tube. Replace the stopper and stir gently until alI the unknown has dissolved. Remove the water bath, dry the test tube with a towel, and insert the test tube in a wide- mouth glass bottle containing a crumpled piece of paper. Record the tem- perature every 30 s until all the naphthalene has solidified. Clean up as described in Part B above.

REVIEW

QUESTIONS

Before beginning this experiment in the laboratory, you should be able to answer the following questions:

1.Distinguish between soh¢te and solvent.

2.List three colligative properties and suggest a rationale for the choice

of the word colligative to describe these properties.

3.Distinguish between volatile and nonvolatile substances.

4.What effect does the presence of a nonvolatile solute have upon (a)

the vapor pressure of a solution, (b) the freezing point, and (c) the boil- ing point?

5.What is the molality of a solution that contains 1, urea (molar mass

= 60 amu) in of benzene?

6.What is supercooling? How can it be minimized?

7.Calculate the freezing point of a solution containing 6.50 g of benzene

in of chloroform.

8.A solution contaLning of an unknown substance in of

naphthalene was found to freeze at 75.4°C. What is the molar mass of the unknown substance?

How many grams of NaNO

3 would you add to of HaO in order to prepare a solution that is 0,200 molal in NaNO3?

Define

molality and molority.

*Instructor Note: cyclohexane may be substituted for naphthalene in this part of this experi- .,ment, but it must be cooled with an ice bath and kept away from flames.

Name

Da~eLaboratory Instructor

Desk

REPORT SHEET

Colligative Properties:Freezing-Point Depressionand Molar Mass

EXPERIMENT

18

1.Weight of test tube + naphthalene

2.Weight of test tube

3.Weight of naphthalene

g g g

4.Weight of paper + sulfur or unknown

5.Weight of paper

6.Weight of sulfur or unknown

g g g

Cooling-curve data

Pure naphthaleneNaphthalene + sulfur or unknown

7.Freezing point of pure naphthalene, from cooling curve

8.Freezing point of solution of sulfur or unknown in naphthalene

9.Molality of sulfur or unknown (show calculations)

195

196 Report Sheet ° Colligative Properties: Freezing-Point Depression and Molar Mass

10. Molar mass of sulfur or unknown (show calculations) ~

HAND IN YOUR COOLING CURVES WITH YOUR REPORT SHEET.

QUESTIONS

1. What are the major sources of error in this experiment?

2.Suppose your thermometer consistently read a temperature 1.2° lower than the correct temperature

throughout the experiment. How would this have affected the molar mass you found?

3.If the freezing point of the solution had been incorrectly read 0.3° lower than the true freezing point, would

the calculated molar mass of the solute be too high or too low? Explain your answer.

4.Arrange the following aqueous solutions in order of increasing freezing points (lowest to highest temper-

ature): 0.10 m glucose, 0.10 m BaC12, 0.20 m NaC1, and 0.20 rn Na2SO 4.

5. What mass of NaCI is dissolved in of water in a 0.050 m solution?

6. Calculate the molalities of some commercial reagents from the following data:

HC1HC2H302NH3(aq)

Molar mass (amu)36.46560.0517.03

Density of solution (g/mL) 1.191_~)50.90

Weight %37.299.828.0

Molarity12.I17.414.8

Laboratory Experiments

197

7.A solution of of para-dichlorobenzene (a clothes moth repellant) in of cyclohexane freezes at

1.05°C. What is the molar mass of this substance?

COOLING CURVE FOR PURE NAPHTHALENE

COOLING CURVE FOR SOLUTION OF SULFUR OR UNKNOWN IN NAPHTHALENEquotesdbs_dbs17.pdfusesText_23

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