Final answer:
FeCl₃ aqueous solution has a lower vapor pressure compared to an equal molar concentration of C₁₂H₂₂O₁₁ aqueous solution because iron chloride dissociates into more solute particles, thus producing a greater effect on vapor pressure reduction according to colligative properties and Raoult's law.
Step-by-step explanation:
A 0.85 m FeCl3 aqueous solution has a lower vapor pressure than that of a 0.85 m C12H22O11 aqueous solution because FeCl3 dissociates into more solute particles, which results in a larger reduction in vapor pressure due to colligative properties. This phenomenon can be explained using Raoult's law which states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent in the solution. As FeCl3 dissociates into four separate ions (one Fe3+ ion and three Cl- ions), the number of solute particles is effectively more than that in an equal concentration of C12H22O11, which is a non-electrolyte and does not dissociate. This higher concentration of solute particles reduces the vapor pressure more significantly according to the colligative properties of solutions.