The molecular mass of a substance was determined by the freezing point depression technique. The minimum data required for the determination are
a. delta Tf and Kf only.
b. delta Tf , Kf and mass of solute.
c. delta Tf , Kf , mass of solute and mass of solvent.
d. delta Tf , Kf , mass of solute, mass of solvent and identity of solvent.
e. delta Tf , Kf , mass of solute, mass of solvent and identity of solvent and solute
?
delta T = dT = i*Kf*m
m = mols/kg solvent
moles solute = g solute/molar mass solute
Working backwards, this is what we must have.
moles = g solute/molar mass solute.
If we are to solve for molar mass solute, we must have g solute in order to know moles.
Next we have m = moles/kg solvent
We have moles now, so to solve for m we must have kg solvent.
Next we have delta T = Kf*m
We now have m, we must have
Kf and delta T
I have typed in bold those that we must have. In addition, we must know i in dT = i*Kf*m. If we know the identity of the solvent and identity of the solvent, we usually know i, the van't Hoff factor.
so its e?
The freezing point depression technique is a method used to determine the molecular mass of a substance. In order to carry out this calculation, the following minimum data are required:
a. delta Tf and Kf only.
Delta Tf (change in freezing point) is the difference between the freezing point of the pure solvent and the freezing point of the solution containing the solute. Kf (cryoscopic constant or freezing point depression constant) is a characteristic property of the solvent, which indicates how much the freezing point is lowered for a given concentration of the solute.
By knowing these two values, delta Tf and Kf, it is possible to calculate the molecular mass of the solute using the formula:
Molar mass of solute = (Kf * mass of solvent * molality) / delta Tf
However, to obtain a more accurate calculation, it is recommended to have additional data:
b. delta Tf, Kf, and mass of solute: Knowing the mass of the solute allows for a more precise calculation of the molar mass.
c. delta Tf, Kf, mass of solute, and mass of solvent: Including the mass of the solvent is important because the concentration of the solute affects the degree of freezing point depression.
d. delta Tf, Kf, mass of solute, mass of solvent, and identity of solvent: The identity of the solvent is necessary because the value of Kf is specific to each solvent.
e. delta Tf, Kf, mass of solute, mass of solvent, identity of solvent, and identity of solute: Knowing the identity of both the solvent and solute is useful for additional calculations, such as determining the molality (moles of solute per kilogram of solvent) which is necessary for the accurate determination of molecular mass.
Based on this explanation, the correct answer is:
c. delta Tf, Kf, mass of solute, and mass of solvent.