Van't Hoft factor
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In physical chemistry, the Van't Hoft factor i is the number of moles of solute actually in a solution in water, per mole of solid solute added.
For example, glucose has a Van't Hoft Factor of 1 because one mole of glucose dissolved in water will result in one mole dissolved. The Van't Hoft Factor of sodium chloride is 2 because one mole will dissociate completely into one mole of Na+ and one mole of Cl- for a total of 2 moles. Substances that dissociate partially, such as acetic acid have a fractional Van't Hoft factor, which is an average of the different dissociation states, weighted for the occurrence of each. That for acetic acid is just over 1 (representing the equlibrium between HC2H3O2 and the ions H+ and C2H3O2-)