Determination of the Molar Mass of an Unknown Compound
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Procedure
1. There are 6 unknowns. Clean and dry a 250 mL beaker and then pour into the beaker 100 mL of one of the unknowns and record both the identification letter of the unknown and the mass of the unknown dissolved in 500 mL of solution. Label the beaker “A”.
2. Clean and dry a second 250 mL beaker and pour into it about 100 mL of 0.1000 M NaOH solution. Label the beaker “B”.
3. Obtain a burette and use three 5 mL portions of the NaOH solution to rinse the burette, including the tip.
4. Pour about 10 mL of the unknown acid solution into a clean and dry 100 mL beaker. Use the liquid in this beaker to rinse out a 25.00 mL pipette twice.
5. Fill the burette with the NaOH solution and drain some liquid to ensure the tip is also filled with liquid. Bring the starting volume down to 0.00 mL and record the initial volume.
6. Pipette exactly 25.00 mL of unknown acid into a clean, but not necessarily dry, 250 mL flask. Add 3 or 4 drops of phenolphthalein indicator.
7. Titrate the unknown acid with the base in the burette until the indicator remains a faint pink colour when the flask is swirled. Record the final volume of base added from the burette.
8. Repeat steps 5 to 7 a second time. If the total volume of base added is within 0.10 mL of the first time, you are finished. If not, repeat the titration a third time.
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Questions
1. Calculate the average volume of the NaOH you used, discarding any value that lies too far from the average of the others.
2. Using the examples you have learned in class, calculate the molar mass of the unknown you selected. Show all the steps in your calculation in a clear manner, explaining what you are calculating at each point.
3. What effect would each of the following have on the value for the molar mass you calculated?
a) The acid solution was accidentally diluted with extra water before you took the acid sample that you used in your titration. Explain your answer in a complete manner.
b) The NaOH solution was accidentally diluted with extra water before you took the NaOH sample that you used in your titration. Explain your answer in a complete manner.
