In BHT Freezing Point Depression 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 answers.
The thermometer used actually read 1.4°C too high.
Some of the solvent was spilled before the solute was added.
Some of the solute was spilled after it was weighed and before it was added to the solvent.
Some of the solution was spilled after the solute and solvent were mixed but before the freezing point was determined.
I made a typo in equation 1. I should have written
delta T = (Tfinal-Tinital) = Kf*m but that doesn't change any of my answer or the logic.
1. The thermometer used actually read 1.4°C too high:
This error would result in the calculated molecular mass of the solute being too low. This is because the measured freezing point depression would appear larger than it actually is, leading to a smaller calculated molality of the solute. Since the molecular mass is directly proportional to the molality, a smaller molality would result in a smaller molecular mass calculation.
2. 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 by the colligative properties, which are dependent on the total concentration of particles in the solution. Since the solvent is not the solute being measured, spilling some of the solvent before adding the solute would not impact the concentration or colligative properties.
3. Some of the solute was spilled after it was weighed and before it was added to the solvent:
This error would result in the calculated molecular mass of the solute being too high. Spilling some of the solute would result in a lower amount of solute being added to the solvent. This would reduce the concentration of solute particles in the solution and therefore reduce the observed freezing point depression. Since the molecular mass is determined based on the colligative properties, a smaller observed depression would lead to a larger calculated molecular mass.
4. Some of the solution was spilled after the solute and solvent were mixed but before the freezing point was determined:
This error would have no effect on the calculated molecular mass of the solute. Once the solute and solvent are mixed, the solution is expected to be homogeneous, meaning the concentration and colligative properties would not be affected by spilling some of the solution before determining the freezing point. As long as enough solution remains to accurately measure the freezing point depression, the calculated molecular mass would not be affected.
1. The thermometer used actually read 1.4°C too high:
If the thermometer used read 1.4°C too high, it would lead to a calculated freezing point depression that is also too high. This is because the temperature difference between the freezing point of the pure solvent and the freezing point of the solution would be incorrectly measured. As a result, the change in freezing point caused by the solute would be overestimated, leading to an erroneously higher calculated molecular mass of the solute.
2. Some of the solvent was spilled before the solute was added:
If some of the solvent is spilled before the solute is added, it would have no effect on the calculated molecular mass of the solute. This is because the solvent only serves as the medium in which the solute dissolves, and it does not directly contribute to the determination of the molecular mass. As long as the correct amount of solute is added and dissolved in the remaining solvent, the percentage of the solute in the solution will remain the same, allowing for an accurate calculation of the molecular mass.
3. Some of the solute was spilled after it was weighed and before it was added to the solvent:
If some of the solute is spilled after it is weighed and before it is added to the solvent, it would lead to a calculated molecular mass that is too low. This is because the amount of solute added to the solvent would be less than the intended amount, resulting in a lower concentration of the solute in the solution. As a result, the observed freezing point depression would be smaller than expected, leading to an underestimation of the molecular mass of the solute.
4. Some of the solution was spilled after the solute and solvent were mixed but before the freezing point was determined:
If some of the solution is spilled after the solute and solvent are mixed but before the freezing point is determined, it would have no effect on the calculated molecular mass of the solute. This is because the freezing point depression will have already taken place due to the presence of the solute in the solvent. Therefore, as long as the remaining solution is used to determine the freezing point, the calculated molecular mass of the solute will be unaffected by the spillage.
Here are the three equations used to calculate the molar mass and I've labeled them eqn 1, 2 & 3. Remember that making the numerator of a fraction too large makes the result too large and making the numerator too small makes the result too small. Also, making the denominator of a fraction too large makes the result too small and making the denominator too small makes the result too large.
equation 1. delta T = (Tfinal - Tinitial )= Kb*molality
equation 2. moles solute = molality x kg solvent
equation 3. molar mass solute = grams/moles.
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 answers.
The thermometer used actually read 1.4°C too high.
See eqn 1. Tfinal will be 1.4 degrees too high and Tinitial will be 1.4 degrees to high but delta T will be the same so the calculated molar mass will be unchanged
Some of the solvent was spilled before the solute was added.
See eqn 2 first. You weigh the solvent but spill some before the solute is added; therefore, the weight of the solvent entered into the calculations is too much (too high) which makes moles from eqn 2 too high, which makes molar mass from eqn 3 too small.
Some of the solute was spilled after it was weighed and before it was added to the solvent.
I'll leave this one and the next one for you. Go through step by step.
Some of the solution was spilled after the solute and solvent were mixed but before the freezing point was determined.
Post your work if you get stuck or if you want someone to check your answers to the last two.