CHEM 122 Lecture Notes - Lecture 7: Weighing Scale, Molar Mass

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9 Aug 2016
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Lab 7
Colligative Properties: Freezing Point Depression
Purpose: To determine the molar mass of an unknown solid by measuring its effects on the freezing point of a
solvent.
Experimental Procedure: Please obtain the small vial from your TA. It is clean and dry, do not wash it. All
experiment is to be done underneath the ventilation hood.
1. Login to the computer. Start Logger Pro. Open experiment file “Freezing Pt. Depression.”
2. Set up freezing point measurements.
Add one scoop of salt rock and 50 mL of the water to the foam cup and place in a 400 mL beaker.
Fill the cup with ice and stir again to suspend the salt. Then insert the thermometer provided by
your TA; be careful not to poke a hole in the Styrofoam cup. Leave a stirring rod in the ice/salt
bath and stir frequently throughout the experiment. Place the rubber stopper in the bottom of
the large vial, and clamp the vial into the salt bath.
Mark outside the small vial at a height of 3 cm up from the bottom. Place on weighing scale.
Pour cyclohexane into the vial up to the mark. Return to the balance and re-weigh.
Insert the temperature probe and stir.
3. A – Determination of the Freezing Point to Cyclohexane.
When the temperature probe reaches to 10 degrees C, place the small vial with the temperature
probe into the larger vial that is clamped in the ice/salt bath.
Click on collect to start collection of data.
Stir constantly and record the temperature for 10 minutes.
Storing and Saving Data: Store latest run from the experiment menu. Get the data analysis from the graph’s
statistics and record onto your DO sheet.
4. B – Freezing Point Depression
You are to require three samples of the unknown solute, each weighing about 0.10 grams. Do not
weigh them out at once.
If the temperature of your bath has risen about -3 or -4 Celsius, drain water and add more salt
and ice.
5. Trial 1: Weigh out the first sample of the unknown solute using the top loading balance an record the
exact weigh on the DO sheet.
Place temperature probe into the solution. Stir to dissolve the solid, and warm or cool the
mixture until the temperature is about 10 degrees.
Click collect to being data collection.
When the run has finished choose store latest run from the menu and save to desktop.
Data Analysis must be Linear Fit line.
From Analyze menu also choose liner fit line. The graph should now have two best fit line displayed.
Record the freezing time on your DO sheet.
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