Q1(g). Van't Hoff factor

Answer in brief

i=1+(ν-1)α, where one formula unit produces ν particles on complete dissociation.

Key points

• The factor i is the observed colligative effect divided by that predicted for the same analytical concentration of an ideal nondissociating solute.
• Start with one mole of solute and let the fraction dissociated be α. Undissociated solute contributes 1-α moles of particles and the ions contribute να.
• The total is i=(1-α)+να=1+(ν-1)α, giving α=(i-1)/(ν-1). For a 1:1 electrolyte, i=1+α.
• This particle-count interpretation assumes ideal dilute behaviour.