The mol fractions (Xbenzene = 0.25) & (Xtoluene = 0.75) in a solution are given at 25C. The densities at are (ρbenzene = 0.876 g/mL) & (ρtoluene = 0.867 g/mL). What is the molal concentration of both?

Answer 1

Molal concentration = moles of solute / kg solvent

So, we need to find (calculate) moles of benzene and toluene, and the total mass of solution in kg.

Moles of benzene and toluene:

Let us assume that we have a total of 1 mol (benzene + toluene). Since the X_benz = 0.25 and X_tol = 0.75, that means we have 0.25 mol benzene and 0.75 mol toluene.

Mass of solvent:

Since toluene is present in the larger amount, it is considered as the solvent.

0.75 mol toluene x 92.14 g / mol = 69.11 g of solvent = 0.06911 kg of solvent (toluene)

Molality of each compound:

[benzene] = 0.25 mol / 0.0691 kg = 3.62 m

[toluene] = 0.75 mol / 0.0691 kg = 10.9 m