Molecular Mass by Freezing point Depression (Preliminary Lab)
The following errors occurred when the above experiment was carried out. How would each affect the calculated molecular mass of the solute (too high,too low, no effect)? Explain your answer.
(a) The thermometer uses actually read 1.4C too high.
(b) Some of the solvent was spilled before the solute was added.
I gave you the answer for a from your first post.
For #2.
(1) delta T = Kf*m
(2) molality = moles solute/kg
(3) molar mass = grams solute/# moles.
So if some of the solvent is spilled before the solute is added, that will mean the molality is too large (equation 2). So when we measure delta T (equation 1) we see delta T too large. Now we go backwards. Delta T too large, m is too large(eqn 1), calculated moles is too large (eqn 2), and (from equation 3), molar mass is too small (because the denominator is too large). Check my thinking.
(a) The thermometer reading is 1.4°C too high: This error would have an effect on the calculated molecular mass of the solute, making it appear too high. Freezing point depression is directly proportional to the molality (the amount of solute in moles divided by the mass of the solvent in kilograms), which in turn is directly related to the molecular mass of the solute. The erroneous high thermometer reading would lead to a lower observed freezing point depression, resulting in a higher calculated molecular mass for the solute. Therefore, the calculated molecular mass would be too high.
(b) Some of the solvent was spilled before the solute was added: This error would have no effect on the calculated molecular mass of the solute. The molecular mass is determined based on the colligative properties of the solvent in the presence of the solute. The amount of solvent spilled before the addition of the solute does not affect the concentration of the solute in the remaining solvent. As long as the correct amount of solute is added to the remaining solvent, the freezing point depression and, consequently, the calculated molecular mass of the solute would not be affected. Therefore, this error would have no effect on the calculated molecular mass.
(a) The error in the thermometer reading (1.4°C too high) would affect the calculated molecular mass of the solute. Specifically, it would lead to an erroneous higher value for the freezing point depression.
To understand why this happens, let's first examine the principle behind the freezing point depression method. According to Raoult's law, when a non-volatile solute is added to a solvent, it lowers the freezing point of the solvent. The extent of this depression in the freezing point is directly proportional to the concentration of the solute.
Now, in the experiment, the freezing point depression is measured using a thermometer. If the thermometer reads 1.4°C too high, it would suggest that the freezing point depression is greater than it actually is. This means that the calculated concentration of the solute would appear higher than it actually is. Since the molecular mass of the solute is directly related to its concentration, an overestimated concentration would lead to an overestimated molecular mass. Therefore, the calculated molecular mass of the solute would be too high.
(b) The spillage of some solvent before the solute was added would not affect the calculated molecular mass of the solute.
In the freezing point depression method, the change in freezing point is primarily determined by the concentration of the solute in the solvent. As long as the correct amount of solute is added, the counted mass of the solvent does not affect the calculated molecular mass of the solute. The volume of the solvent may change due to the spillage, but this would not impact the concentration of the solute or the freezing point depression, which are the key factors for calculating the molecular mass. Hence, the spillage of solvent would have no effect on the calculated molecular mass of the solute.