Question

If benzoic acid (mol. mass = 122) is associated into double molecules when dissolved in benzene and the osmotic pressure of a solution of 5 g of benzoic acid in 100 mL benzene is 5.73 atm at ${10}^{o}C$, what is the percentage association of benzoic acid?

• A. $60\%$
• B. $70\%$
• C. $80\%$
• D. $85\%$

Answer 1

Answer: (C)

$80\%$

$\pi =iCRT=i\frac{n}{V}RT$
$5.73=i\times 5/1220.1\times 0.0821\times 283$
$i=0.6017$
$\alpha =\frac{1-i}{1-1/n}=\frac{1-0.6017}{1-1/2}=0.8$
$\%$ association $=80\%$