The solution contains 1.18 moles of the unknown. What is the molar mass of the unknown compound?

Question

A solution made by dissolving 150.0 g of an unknown compound in 370.0 mL of benzene to make a solution which has a freezing point 18.6 °C lower than that of pure benzene. Kf for benzene is 5.12 °C/m and the density of benzene is 0.877 g/mL. The solution contains 1.18 moles of the unknown. What is the molar mass of the unknown compound?

Answer 1

A similar question was asked like this before...

Based in the information provided we needed to use the equation ∆T = K_fm where m is the molality. 

m = 18.6°C /5.12 °C/m = 3.63 m

Molality = moles of solute /kg solvent  [ m = moles solute/kg solvent] =

You were given 370.0 mL of benzene and the density of benzene. With this information the mass of benzene can be calculated.

g of benzene used = 370.0 mL (0.877 g/mL) = 324.5 g =0.3245 kg solvent

Substituting these values in the equation,

[ m = moles solute/kg solvent] the moles of solute can be calculated

Moles of solute = 3.63 * 0.3245 = 1.18 moles solute [this is how they obtained that value!]

The # of moles of a substance = g of substance /Molar mass

Given 150.0 g of the solute you can calculate the molar mass!

        1.18 moles = 150.0 g/ M. Mass

M. Mass = 150.0 g/1.18 moles  = 127 g/mol

I suggest you review the chapter with this!