if the freezing point of the solution is recorded 0.2 degrees C lower than the actual freezing point, will the molar mass determination for the unknown solid be too high or too low?
it will appear that there are more moles of solute than are actually present
you will be dividing the mass of added solute by too large a number
... this will result in the molar mass being too low
If the freezing point of a solution is recorded 0.2 degrees Celsius lower than the actual freezing point, the molar mass determination for the unknown solid will be too high.
The freezing point depression is a colligative property that depends on the concentration of solute particles in a solution. It is directly proportional to the molality of the solution. In other words, the greater the molality, the greater the freezing point depression.
When determining the molar mass of a solute using freezing point depression, we use the equation:
ΔT = Kf * molality
In this equation, ΔT represents the difference between the freezing point of the pure solvent and the freezing point of the solution, Kf is the cryoscopic constant specific to the solvent, and the molality is the amount of solute in moles per kilogram of solvent.
Since the recorded freezing point of the solution is lower than the actual freezing point, the value of ΔT will be higher than it should be. Consequently, according to the equation, the molality will be calculated as higher than it actually is. This leads to a higher molar mass determination for the unknown solid.
In summary, if the freezing point of the solution is recorded 0.2 degrees Celsius lower than the actual freezing point, the molar mass determination for the unknown solid will be too high.
To determine the effect on the molar mass determination, we need to understand the concept of freezing point depression.
Freezing point depression refers to the phenomenon where the freezing point of a solution is lower than that of the pure solvent. It occurs when a non-volatile solute is added to a solvent, disrupting the arrangement of solvent particles and making it more difficult for them to form a solid.
In this case, if the recorded freezing point of the solution is 0.2 degrees Celsius lower than the actual freezing point, it means that the observed freezing point depression is greater than the true value. This suggests that the actual concentration of the solute is higher than what was measured.
Now, let's consider the relationship between molality (moles of solute per kilogram of solvent) and freezing point depression. According to the equation:
ΔT = K_f ∙ m
where ΔT is the freezing point depression, K_f is the cryoscopic constant of the solvent, and m is the molality of the solute.
Since the freezing point depression is higher than what was recorded, we can assume that the measured molality (m) is lower than the actual value. As a result, the calculated molar mass will be too high.
This discrepancy arises because the molar mass determination is calculated based on the molality of the solution. If the molality is underestimated, the calculated molar mass will also be overestimated.
Therefore, in this scenario, the molar mass determination for the unknown solid will be too high due to the lower recorded freezing point of the solution.