Solution contain 8.3 g of nonvolatile substance dissolved in 1 mol CHCl3 has vapor pressure 511 torr the vapor pressure of pure CHCl3 at the same temp is 526 calculate the mol fraction of the solute and number of mole solute in solution ??

#chi_(B(l)) = 0.0285#
#n_B = "0.0294 mols"#

What would the molar mass of the solute be?

Since you're given a vapor pressure for the solvent and for the solution, use Raoult's law...

#P_A = chi_(A(l)) P_A^"*"#

where #chi_(A(l))# is the mol fraction of solvent in the solution phase, #P_A# is the vapor pressure of the solvent in the context of the solution, and #"*"# indicates pure solvent.

Therefore, the mol fraction of solute is:

#color(blue)(chi_(B(l))) = 1 - chi_(A(l))#

#= 1 - P_A/P_A^"*"#

#= 1 - "511 torr"/"526 torr"#

#= color(blue)(0.0285)#

Knowing the mol fraction of the solvent is #chi_(A(l)) = 1 - chi_(B(l)) = 0.9715#,

#chi_(A(l)) = 0.9715 = "mols solvent"/"mols solute + solvent"#

#chi_(B(l)) = 0.0285 = "mols solute"/"mols solute + solvent"#

Therefore:

#chi_(A(l))/(chi_(B(l))) = 0.9715/0.0285 = "mols solvent"/"mols solute" = 34.07#

And so, the mols of solute are found from the mols of the solvent.

#"1 mol solvent"/"mols solute" = 34.07#

#=> color(blue)(n_B) = 1/34.07 = color(blue)("0.0294 mols solute")#

What is the molar mass of the solute?