The molality of the solution is 3.63 m. What quantity in moles of the unknown compound were dissolved in the solution?

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 molality of the solution is 3.63 m. What quantity in moles of the unknown compound were dissolved in the solution?

Answer 1

Based on the information provided we needed to use the equation ∆T = K_fm where m is the molality. (This is how the molality value was obtained.)

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

You were given 370.0 mL of benzene and the density of benzene 0.877 g/mL. With this information the mass of benzene can be calculated.

Mass 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

Hope this helps!