Determination of Molar Mass

1. Determination of Molar Mass • Use freezing point depression to determine the molality of a solution, which is then used to determine the molar mass of dextrose • ΔTf = Kfp x molality x i • The solute used is a molecular solid. What does that imply about the van’t Hoff factor?

2. Pre-Lab Write-up • Title • Purpose • Procedure • Data table

3. Procedure • Put on apron and goggles On a balance, tare a small (50,100, 150-mL) beaker. Add about 50-mL of D.I. water (40 mL if using a 50 mL beaker) then determine the mass of the water. Be sure to record accurately and to the proper precision (see next slide)

4. Preparing the solution of dextrose. • Tare a weigh boat and add about 10 grams (8 if using 40 mL of H2O) of dextrose and determine the mass. • Add the dextrose to the beaker with water a little bit at a time and stir, stir, stir! • Continue stirring until all of the dextrose has dissolved.

5. Finding Your Calibrated Freezing Point Determine the freezing point of water by placing some ice and a small amount of water in a beaker. Stir to make a mixture and record the temperature. Remember it should be close to 0oC!

6. Using a hammer crush enough ice in a towel to fill a 600 mL beaker about 2/3 full. Add a layer of the ice and then sprinkle salt on top of the layer. Repeat until beaker is about 2/3 filled with layers of ice and salt. Making the Salt-Water Slurry • Place the beaker containing your dextrose solution in your slurry – make sure that the slurry level is above the dextrose solution. • Distribute the temperature evenly by spinning the beaker.

7. Determining the Freezing Point • Place the small beaker in the ice/salt bath and gently rotate it. • Place the temperature probe in the dextrose solution to measure the temperature. • Add a small seed crystal of ice when the temperature reaches 00C. • Continue to swirl the small beaker until there are definite signs of crystallization. • RECORD THE TEMPERATURE

8. Clean Up • Clean up • Return the Vernier lab equipment to the proper place • Dispense the solutions down sink • Clean the glassware.

9. Some time saving ideas Make sure to stir right away so the dextrose will not “clump” on the bottom of your beaker. The mass on the bottom is hard to dissolve. Pack small chips of ice around your beaker and sprinkle with salt to melt the ice and lower the temperature of the bath. Don’t get any of your slurry in your dextrose solution.

10. Calculations • Use the following three equations to determine the molar mass of the solute • Δ T = Kfp x m x i mx Kg solvent= moles g solute/ moles = molar mass Where Kfp is the freezing point depression constant for water and m is the molality of the solution. Also, find percent error: E – T x 100 T Experimental: as determined from your lab data Theoretical: as determined from the formula C6H12O6 and the periodic table.

11. Post-Lab • Calculations • Determine m of solution and from that molar mass of dextrose • Determine % Error Conclusion should include: Summary of experiment What did you do and how did you do it? Summary of calculations What did you find and how? Results What was your new freezing point? What was your molar mass? What is the actual molar mass? What was your percent error? What could have caused your percent error? What would have happened to the freezing point of the solution if your solute would have been sodium chloride instead of dextrose? Explain.