Does vapor pressure vary with the number of particles in solution? What if you have #"1 M"# of #"NaCl"#, vs. #"1 M"# #"KCl"#, vs. #"1 M"# #"CaCl"_2#?
Maybe the following concept map will make things clearer.
These electrolytes generate approximately two, two, and three ions, respectively, in solution for each formula unit of the solute.
#"NaCl"(aq) -> "Na"^(+)(aq) + "Cl"^(-)(aq)#
#"KCl"(aq) -> "K"^(+)(aq) + "Cl"^(-)(aq)#
#"CaCl"_2(aq) -> "CaCl"^(+)(aq) + "Cl"^(-)(aq)#
#"CaCl"^(+)(aq) -> "Ca"^(2+)(aq) + "Cl"^(-)(aq)#
Hence, the molar and molal concentration, and thus the NONIDEAL "mol fraction" (will be discussed below), of electrolyte particles originating from
The approximate concentrations of the above solutions are
Refer to this answer for more on osmotic pressure, vapor pressure, and freezing point. All of these depend on the effective concentration of electrolytes in the solution.
RAOULT'S LAW IS ONLY FOR IDEAL SOLUTIONS
The decrease in vapor pressure,
#P_j = chi_(j(l))P_j^"*"#
in terms of mol fractions of solute
#DeltaT_f = T_f - T_f^"*" = -iK_fm#
already account for nonideality using the van't Hoff factor
AN ADJUSTED "RAOULT'S LAW"
Raoult's law can be rewritten in another way that does account for nonideality:
#P_j = a_(j(l))P_j^"*"#
#a_(j(l)) = gamma_jchi_j#is the activity of the solute #j#in the liquid phase.
#gamma_j#is the activity coefficient of the solute #j#.
#chi_(j(l))#is the mol fraction of the solute #j#in the liquid phase.
The activity coefficient
Since mol fraction, molality, and molarity are approximately interconvertible (see the above concept map), we can indirectly say that the "true mol fraction" (the activity) of solute in solution is not simply due to the original solute, such as
So, no, we cannot say that the vapor pressure decrease is to the same extent for solutions of nonvolatile solutes, even if they have the same face-value concentrations.
We must look at their effective concentrations instead, which is not readily-seen trend and might have to be inferred indirectly when it comes to vapor pressure.