to find the freezing point, you use the following formula
fp solution= fp solvent - Δfp solution
fp solvent= 0 C (since the solvent was not stated, you can assume is water which freezing point is 0 degree celsius)
but first, we need to find the depression in freezing point (Δfp solution)
Δfp solution= Kf x i x m, where Kf is constant for the solvent, i is the number of particles produced by the solute and m is the molality of the solution.
Kf= 1.86 for water
i= 4 , becausethe solute breaks into 4 ions, K3PO4---> 3K+ + PO4-3
m= 2.60
Δfp solution= (1.86) (4) (2.60)= 19.3 C
fp solution= 0 - 19.3C= -19.3C
for boiling point, we use a similar formula and way to solve it.
Δbp solution= bp solvent + Δbp solution
bp solvent= 100 C for water
to find the elevation boiling point (Δbp soln) is equal to:
Δbp solution= Kb x i x m
Kb= 0.512
i= 4
m= 2.60
Δbp solution= (0.512) (4) (2.60)= 5.32 C
bp solution= 100C + 5.32C= 105.32 C