Question

Mole fraction of HCl is 0.78. Calculate its molality.

Answer 1

The mole fraction is the ratio of moles of solute of solvent. A mole fraction of 78 means thet for every mole HCl, there are 78 moles of water. Molality is the number of moles of solute per mass of solvant. Since HCl has a molar mass of 36.5 g/mol, we would use dimentional analysis to calculate the molality:
$\frac{0.78mol{H}_{2}O}{1molHCl}.\frac{1molHCl}{36.5gHCl}=0.21\frac{mol{H}_{2}O}{gHCl}$
Usually, molality is given in mol/kg, so this is 21 mol/kg