Counting the Number of Molecules by Freezing Point …



Counting the Number of Molecules by Freezing Point Depression

One of the most important and tricky measurements to make in a molecular science is to count how many molecules are present in a sample. Last semester you counted the number of acid molecules by titration. A powerful technique for counting molecules (or rather moles of molecules) is from the colligative properties of solutions. This is a property that changes linearly with the addition of a solute. This week in lab we will use the colligative property of freezing point depression to measure the amount of solute added to solvent. The linear relationship between change in freezing point and concentration of solute is given by the equation:

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From the measurements of mass and number of moles the molar mass on an unknown compound will be measured. This topic is covered in section 12.6 of the Chang text.

Iridium platinum thermisters can measure temperature to an accuracy of 0.001 (C. This allows the solvent to be water with a modest freezing point constant of Kf = 1.86 (C/m. The accurate thermometers are used to determine the freezing point depression in urine. A failing kidney will can be diagnosed due to its higher freezing point because not enough salts are being excreted in the patient’s urine. Dairy farms also have their milk regularly checked by freezing point depression to prove that they are not watering down a shipment.

Procedure:

Determine the Freezing Point of t-Butylalcohol (TBA), Kf = 8.1 (C /m.

• After turning in your pre-lab, obtain from your TA a thermometer, stir-bar and unknown sample.

• Obtain a known mass of t-butylalcohol in the test tube. The empty test tube containing your stir-bar should be weighed before adding the t-butylalcohol. If the test tube is placed in a small beaker it will stand-up on the balance. Fill the test tube with 30 to 40 mL of TBA.

• Prepare an ice bath in one of your largest beakers. The ice bath should only be around 10 (C. Any cooler and the sample will freeze too quickly.

• Set-up a ringstand with one clamp to hold the t-butylalcohol test tube and another clamp to hold the thermometer above the stir-bar. Do not overtighten the clamp around the thermometer, the plastic casing will crack. Be sure that the plastic of the thermometer is an inch above the TBA in the test tube. Examine the set-up at the front of the room. This set-up should allow you to move the test tube and thermometer into and out of the water bath.

• Adjust the height of the test tube and thermometer so they are just above the mechanical stirrer.

• From the front of the room obtain 300 – 400 mL of water in a large beaker.

• Place the test tube of t-butyl alcohol held by the clamp into the warm water so that the temperature of the alcohol rises above 40 (C.

• Create a data table in your lab notebook that has a column for time and a column for temperature. You will record the temperature of TBA as it cools every fifteen seconds.

• Place the ice bath on the stirrer plate.

• Work hard to make sure the metal of the thermometer is in the center of the test tube.

• Begin cooling the t-butylalcohol (while recording the temperature) by moving the sample from the warm water to the ice bath. Turn on the stir motor so that the stir-bar is spinning.

• The temperature will drop until the liquid begins to freeze. At this point the temperature will remain constant until the sample is completely solid. The temperature will then begin to drop as the solid is cooled. Record temperature data for approximately 8 minutes.

• After you have recorded a few temperature measurements that show the solid cooling, you can stop recording temperature.

• Replace the ice bath with the warm water bath on the stir plate. Observe the temperature as the solid begins to melt. Record the solution temperature when there is clearly both liquid and solid present in the test tube. How close is this temperature to the melting point you recorded while cooling the mixture?

• Determine the freezing point of pure TBA. The freezing point is the set of temperatures that remain constant during the cooling cycle. If you have difficulty determining a melting point from the cooling data, refer to the graphical method of determining freezing points given at the end of the lab.

Measurement of Freezing Point Depression for an Unknown Solid.

• After the TBA has melted, remove the test tube from the clamp and dry the sides of the test tube. Reweigh the test tube. Some TBA will have evaporated. Use the average of your two TBA masses in your calculations.

• Your unknown sample vial contains 2 grams of unknown. Place half of your unknown solid in the TBA test tube. (Be careful not to get much solid on the side of the test tube.) Record the mass of the test tube with the additional solid and perform the subtraction necessary to determine the mass of solid in solution.

• Be sure that all of the solid dissolves in the TBA, including any stuck on the sides of the test tube. Placing the test tube back in the clamp and using the stir bar will make dissolving easier.

• Warm the TBA solution to above 40 (C and then repeat the cooling process, recording the solution temperature every 15 seconds. The melting point of TBA with a gram of unknown should be lowered by about 2 (C.

• After freezing the solution, switch to the warm water bath and observe the temperature change of the solution as it warms.

• From these measurements determine the freezing point of the solution and (T, the freezing point depression.

• Remove the TBA solution from the clamp and dry the outside of the tube before measuring the mass. None of the solid will evaporate, so any change in mass is due to evaporational loss of TBA.

• To make a second measurement of the freezing point depression of the unknown solid, add the remaining solid from the sample vial to the TBA solution, being careful to keep the solid from the sides of the test tube. Mass the test tube to determine the total amount of unknown solid in the solution.

• Make a new melting point determination of this more concentrated solution, by repeating the procedure above.

• After making the new melting point determination, and finding (T for the second trial, remove and dry the TBA test tube. Weigh the test tube to determine the amount of TBA lost due to evaporation.

• When the experiment is complete, the TBA solution is to be poured into the TBA waste container, but not the stir bar. Obtain the large stir bar from the front of the room and place it at the mouth of the test tube to catch the small stir bar as the TBA solution is poured into the waste container. Wash the test tube and stir bar. Return the stir bar to your TA.

• Disassemble the rest of the freezing point apparatus. Unclamp the thermometer and wipe it down with a damp paper towel. Replace the thermometer cap and return it to your TA, along with your empty sample vial. The clamps and ring stand should be returned to their proper laboratory location.

Calculations to Determine the Molar Mass of the Unknown Solid.

• From the two trials make two separate measurements of the molar mass of the unknown solid. Recall that the molar mass has units of grams/mole. The grams of sample are quickly obtained from your mass measurements. The moles of unknown solid are obtained from the freezing point depressions.

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This relationship, between change in freezing point and solute concentration, allows you to determine the concentration of the unknown compound in units of molality. This is, of course, not moles of unknown, but rather moles if you had 1000 g of TBA. You need to determine the moles present in your mass of TBA. It might be surprising the small number of moles you are able to measure with this technique.

To report the molar mass of the unknown solid, take an average of the two molar mass measurements. Report the uncertainty as half of the difference between the two mass measurements (ex. if Trial # 1 gives a mass of 240 g/mol and Trial # 2 gives 256 g/mol the average should be reported as 248 ( 8 g/mol).

• After completing your calculations of molar mass, turn in the copies of your laboratory notebook pages and wipe down your lab table with a wet paper towel.

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