Question

Explain how each of the following errors Part B would affect the calculation of the molar mass of the unknown. (which would have been Cyclohexane ,C6H12, molar mass=84.2, boiling point=80.7)

(4): 0.3703g

(6): Temp of vapor in flask 102C 375.15K

(7): Barometric Pressure- 30.35in * 2.54cm/1in * 10mm/1cm= 770.89 --> 1.01 atm

(8): Volume of Erlenmeyer flask- 147 mL 0.147 L

M= (step4)(0.0821L atm/mole)(6) / (step 7)(step 8) = 76.8 g/mol <---that should have been 84.2 if we would have done everything right

a. heating was stopped before all of the liquid in the flask had evaporated

(step 10. Observe the contents of the hot flask: The flask contains more liquid than is required to fill the flask with vapor. As the liquid in the flask begins to vaporize, the liquid level will decrease and excess vapor will escape through the narrow tip of the pipette. At approximately 2-minute intervals, loosen the utility clamp and raise the flask out of the boiling water bath, dry the outside of the flask with a paper towel, observe the amount of liquid present (looking at both the flask walls and the stopper assembly), then lower the flask back in the boiling water bath. When no more liquid is apparent inside the flask or stopper assembly, continue to heat for an additional 5 minutes. At this point, it is assumed that the Erlenmeyer flask is filled with only vaporized unknown.)

b. water remained on the outside of the flask and stopper assembly when the mass of the condensed liquid was determined

(step 12 Remove the flask from the cool water bath, and thoroughly and efficiently dry the outside of the flask and stopper assembly, making sure that no water remains or condenses on the outside of the flask)

c. The volume of the erlenmeyer flask was assumed to be 125-mL and not measured as directed

(step 14 Remove the stopper assembly and discard the unknown liquid in the provided organic liquid waste container. To determine the total volume of the flask, fill the flask to the top with water and reinsert the stopper assembly - some water will spill out. Remove the stopper and pour the water that filled the flask into a graduated cylinder of appropriate volume. Record the volume of water in the flask on the Report Sheet (8).

Answer 1

Determination of molar mass of a gas

a) If heating was stopped before all the liquid has been vaporized, the mass of vapor would be less, the molar mass would thus be higher than the actual value.

b) Water condensed on the outside of the flask would increase the mass of liquid taken (with error), so the molar mass calculated would turn out to be lower than actual value.

c) If the volume of flask is not accurately measured, the molar mass calculated would be higher than the actual value.