A lab experiment to find the molar mass of an unknown substance through the calculation of its freez
Molality is moles of solute per kilogram of solvent.
Start collection, place the test tube into the bath, and stir continuously at a constant rate. At the freezing point, the kinetic energy of the particles becomes low enough for inter-particle forces to lock the particles into fixed positions in the solid crystal structure. Measuring the Freezing Point of Cyclohexane Dispense Using your beaker tongs, place the beaker of hot water onto the lab bench well away from the test tube.
Move the test tube that contains the solution into the ice-water bath.
Lab 12 molar mass and freezing point
Once the liquid has completely frozen, only then will the temperature start to fall again. As the temperature of the water bath reaches the melting point of the PDB, it will begin to melt. Calcium chloride is preferred to sodium chloride for treating icy roads because of the effects of freezing point depression. Therefore, the temperature at which the solution freezes is lower than that of the pure solvent. Note the temperature at which these crystals first start to appear. When the freezing point is reached, solid will begin to form, and the temperature will tend to hold steady until the sample is all solid. Now that you understand the phenomenon of freezing point depression, let's go through a procedure for determining the molar mass of an unknown solute from freezing point temperatures. Fill a mL beaker about one-third full of water, and add ice until the beaker is three-fourths full. Carefully transfer another 2 grams of the unknown solid into the large test tube, taking great care that none of the unknown sample is spilled during this process. Avoid allowing compound to adhere to the test tube.
The sample with the higher mass fraction of sulfur was completely liquid at the temperature of the experiment, whereas the sample with less sulfur was still partially solid.
This demonstrates that with increased impurities, in this case sulfur, the observed melting point is lower than for the pure solid.
Using freezing-point depression to find molecular weight
It is important that no crystals remain. This phenomenon is called freezing point depression. To calculate the molar mass of the unknown compound, the freezing point depression constant of cyclohexane, the mass of solute and solvent used, and the difference in freezing temperatures are all needed. The computer acquires a temperature reading every second. This is shown in Figure 3. Freezing point depression is a colligative property, meaning it is only affected by the ratio of solute to solvent particles, and not their identity. Part 3: Determining the Freezing Point of PDB with about 4 g Unknown Solute Weigh your unknown sample and its container on the electronic balance again, and record this mass on your data sheet. Caution: Cyclohexane is a flammable solvent. Add this mass to the mass of unknown transferred in Step 8 to obtain the total mass of unknown added to the large test tube. Use your utility clamp to clamp the large test tube containing the PDB to the ring stand as shown in Figure 4 we shall add the thermometer and stirrer presently. You've just watched JoVE's introduction to using freezing point depression to determine the identity of an unknown compound. For this reason, ice and rock salt are combined in the outer container of an ice cream maker in order to achieve a temperature low enough to freeze the mixture enclosed in the inner container.
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