Question

In case a solute associates in solution, the van’t Hoff factor is :

• A. $i>1$
• B. $i=1$
• C. $i<1$
• D. None of these

Answer 1

The correct option is C $i<1$

if a fraction $\alpha$ of $n$ moles of solute associate to form one mole of an n-mer (dimer, trimer, etc.), then

$i=1-(1-\frac{1}{n})\alpha .$

For the dimerisation of acetic acid in benzene

$2C{H}_{3}COOH\rightleftharpoons {\left(C{H}_{3}COOH\right)}_2$

2 moles of acetic acid associate to form 1 mole of dimer, so that

$i=1-(1-\frac{1}{2})\alpha =1-\frac{\alpha }{2}.$

For association in the absence of dissociation, the van 't Hoff factor is: $i<1$

Hence, the correct option is $\text{C}$