Question

A)What is the van't hoff factor for Mg₃(PO4)₂ in an ideal solution?

B)What is the vapor pressure in atm of a solution at 25 ^oC produced by dissolving 115.4 g of dextrose (C₆H₁₂O₆) in 357.4 g of water? Vapor pressure of water at 25 ^oC = 0.0313 atm

Answer 1

(A)

van't Hoff factor (i) is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.

for an ionic compound, Van't off factor can be expressed as number of ions produced (on dissolving) per molecule of the compound.

i.e. it dissociates to give 5 ions per Mg3(PO)4 molecule.

In ideal solution (supposedly infinite dilution) there would be no ion pairing and van't hoff factor is equal to the ions produced per molecule of compound. therefore, i = 5

(In real solutions there would be some ion pairing because of interactions between oppositely charged ions and they would be considered as one particle (entity). therefore, for dissociative solutes, actual Van't hoff factor will be less than that of calculated for ideal solution condition. )

(B)

according to Raoult's law,

partial pressure of a component in an ideal solution (mixture) = mole fraction of the component * vapor pressure of pure component.

first calculate,

moles of water (n_water) = 357.4 g / 18 g/mol

= 19.85 mol

moles of dextrose (n_dextrose) =  115.4 g / 180.2 g/mol

=0.64 mol

partial pressure of water in the solution = x_water * vapor pressure of water

= 0.97 * 0.0313

= 0.0303 atm