Does anyone know how to do this problem, i can't seem to solve it.
If 39.25 g of a substance is dissolved in 155.5 g of benzene solvent, the freezing point of the solution is 0.08 oC. Calculate the apparent molar mass (g/mol) of substance.
delta T = Kf*molality.
molality = moles solute/kg solvent.
moles = g/molar mass.
Post your work if you gst stuck.
Use equation 1 to solve for molality.
Use equation 2 to solve for moles solute.
Use equation 3 to solve for molar mass.
so this is what i did,
0.08C = (5.065Ckg/mol)(x/0.1555) which is 0.002456071.
i then took 39.25g/0.002456071 which is 15980 g/mol.
This seems to big for molar mass of a substance
Mistake 1. delta T is not 0.08. The problem states that the freezing point of benzene is 0.08 C. You must have or must look up the normal freezing point of benzene and subtract 0.08 to arrive at delta T.
#2. I found 5.12 for the Kf for benzene and the units are oC/m. I don't understand the units you have but the number you have used is not that much different than what I used..
I ran through the problem VERY roughly and obtained something in the neighborhood of 240 for the molar mass.
To calculate the apparent molar mass of a substance, you need to use the freezing point depression equation:
ΔT = Kf * m
where:
ΔT is the change in freezing point
Kf is the cryoscopic constant (freezing point depression constant) of the solvent
m is the molality of the solution
First, convert the mass of the substance and the solvent into moles:
Moles of substance = mass of substance / molar mass of substance
Moles of benzene = mass of benzene / molar mass of benzene
Then, calculate the molality of the solution:
Molality (m) = moles of substance / kg of solvent
= (moles of substance) / (mass of benzene / 1000)
Next, determine the change in freezing point (ΔT) by subtracting the freezing point of the pure solvent from the freezing point of the solution:
ΔT = freezing point of pure solvent - freezing point of solution
Finally, rearrange the freezing point depression equation to solve for the molar mass of the substance:
molar mass of substance = (ΔT * Kf) / molality
Substitute the given values into the equation and solve for the molar mass of the substance.