I just wanted to see if my answers made sense.
1. What effect would each of the following have on your calculated value of Kf (too high, too low, or no
effect)? In each case explain your answer.
a) There was a small bubble in the thermometer, causing all the temperature readings to be erroneously
high.
The Kf would be too high. Because the bubble was giving off higher temperature.
b) A student used a new thermometer for Part II, but failed to notice that some of the naphthalene from Part I was sticking to the old thermometer.
There would be no effect. The thermometer was not directly in contact with the solution.
c) After melting the naphthalene, small pieces of rubber from the stopper are seen floating in the solution.
No effect. the rubber can simply be taken out.
2. A student decided to use the same weighing boat he used to measure the biphenyl in Part II for
measuring his unknown in Part III. However, he failed to notice that a small amount of biphenyl was still
in the boat, which was transferred to the test tube with his 1g of unknown. What effect would this have on
each of the following (too high, low, or no effect)? In each case, explain your answer.
a) The measured delta Tf
Both would react causing the freezing point to be higher than it would be in reality.
b) The calculated molality of the solution
molal would be higher because there would be a high number of Kg per solvent.
c) The calculated molar mass of the unknown
Molar mass would be unaffected. Although i am not sure why.
To determine the effect of a small amount of biphenyl in the weighing boat on the calculated molar mass of the unknown in Part III, we need to understand how the molar mass is determined in cryoscopic measurements.
In cryoscopic measurements, the molar mass of a solute can be calculated using the formula:
Molar mass = (molar mass of solvent * delta Tf) / (Kf * molality of the solution)
a) The measured delta Tf:
The presence of a small amount of biphenyl in the boat would result in the measured delta Tf for the unknown to be lower than it should be. This is because some of the biphenyl would mix with the unknown and decrease the overall concentration of the solute in the solution. Therefore, the measured delta Tf would be lower than the actual value.
b) The calculated molality of the solution:
Since the measured delta Tf would be lower due to the presence of biphenyl, the calculated molality of the solution would be higher than the true value. This is because the molality is calculated by dividing the mass of the solute by the mass of the solvent. In this case, with the measured delta Tf being lower, the calculated molality would be higher, indicating a higher concentration of the solute in the solution.
c) The calculated molar mass of the unknown:
The molar mass of the unknown is determined using the cryoscopic equation mentioned earlier. Since the calculated molality would be higher due to the presence of biphenyl, it would result in a higher value in the denominator of the equation. However, the numerator (molar mass of the solvent * delta Tf) would still remain the same because the biphenyl does not impact the molar mass of the solvent or the measured delta Tf. Therefore, the calculated molar mass of the unknown would not be affected by the presence of biphenyl in the weighing boat.
In summary, the presence of biphenyl in the weighing boat would affect the measured delta Tf and the calculated molality, but it would not have any effect on the calculated molar mass of the unknown.