The van't Hoff Factor
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The van't Hoff factor is the measured ratio of dissolved particles to formula units dissolved, correcting a colligative calculation for ionization.
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Sodium chloride's factor measures near 1.9, close to but below the 2 predicted for complete dissociation, from residual ion pairing.
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A factor visibly below its predicted whole number is not measurement error — it reflects real residual ion-ion attraction in solution.