What is the molar mass of a nonpolar molecular compound if 3.42 grams dissolved in 41.8 grams benzene begins to freeze at 1.17°C? The freezing point of pure benzene is 5.50°C and the molal freezing point constant,K_f,is -5.12°C/m.

Question

Quizplus · Accepted Answer

First, we need to calculate the change in freezing point of the solution compared to pure benzene:

ΔTf = Tf,pure - Tf,soln = 5.50°C - 1.17°C = 4.33°C

Next, we need to calculate the molality of the solution:

molality = moles of solute / mass of solvent in kg

Assuming that all 3.42 grams of the compound dissolved in 41.8 grams of benzene, the mass of solvent is:

mass of solvent = 41.8 g / 1000 g/1 kg = 0.0418 kg

Using the freezing point depression equation:

ΔTf = KSf * molality

We can solve for molality:

molality = ΔTf / KSf = 4.33°C / (-5.12°C/m) = -0.846 m

Note that molality is negative because the freezing point depression occurs when solute is added to the solvent.

Now we can solve for the moles of solute:

moles of solute = molality * kg of solvent = -0.846 m * 0.0418 kg = -0.0354 mol

Again, the negative sign indicates that the compound is the limiting reagent and that there is an excess of benzene.

Finally, we can calculate the molar mass of the compound:

molar mass = mass of solute / moles of solute = 3.42 g / (-0.0354 mol) = 96.7 g/mol

Therefore, the correct answer is C) 96.7 g/mol.

What Is the Molar Mass of a Nonpolar Molecular Compound