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Wiki- The van 't Hoff factor 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 most non-electrolytes dissolved in water, the van' t Hoff factor is essentially 1. For most ionic compounds dissolved in water, the van 't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only. Sometime ion pairing occurs in solution. At a given instant a small percentage of the ions are paired and count as a single particle. Ion pairing occurs to some extent in all electrolyte solutions. This causes deviation from the van 't Hoff factor. The deviation for the van 't Hoff factor tends to be greatest where the ions have multiple charges.

i=van't Hoff= the number of particles a formula breaks up into C6H12O6(s) ==>C6H12O6(aq)    (1 mole of particles)       C6H12O6 =1 NaCl(s) ==> Na+(aq) + Cl-(aq)    (2 moles of particles)      NaCl=2 CaCl2(s) ==> Ca2+(aq) + 2Cl-(aq)   (3 moles of particles)    CaCl2=3

The greater the concentration of particles the higher the boiling point or the lower the freezing point.

Regents Questions-Follow the Regents link for the answer

Jan 2009-40 Which sample, when dissolved in 1.0 liter of water, produces a solution with the lowest freezing point?
(1) 0.1 mol of C₂H₅OH
(2) 0.1 mol of LiBr
(3) 0.2 mol of C₆H₁₂O₆
(4) 0.2 mol of CaCl₂

Jan 2010-39 Which solution has the highest boiling point at standard pressure?
(1) 0.10 M KCl(aq)
(2) 0.10 M K₂SO₄(aq)
(3) 0.10 M K₃PO₄(aq)
(4) 0.10 M KNO₃(aq)

Jan 2011-48 The table below gives information about four aqueous solutions at standard pressure.

Which list of solutions is arranged in order from highest boiling point to lowest boiling point?
(1) A, B, D, C    (3) C, D, B, A
(2) A, C, B, D    (4) D, B, C, A

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