# Why calculations based on colligative properties of solutions sometimes do gives abnormal molecular mass values for solute? What are the nature of the abnormalities. 2g of C6H5COOH dissolved in 25g of benzene shows a depression in freezing point equal to 1.62k. Molal depression constant for benzene is 4.9k kg mol-1. What is the Percentage(%) of association of acid, if it forms a dimer in solution?

Naveen Kumar
8 years ago
Dear student as we know that the colligative properties depends on number of solute particles and not on the nature of the particle.
when we dissuolve solutes, there are three possibilities:
1. the number of molecules in the solutionm issame as of intially dissolved
2.the particles dissociate and the particles in the solution, increases
3. two or more molecules ccombine (or associate)and form one particles
The last two cases give you abnormal properties as the number opf particles are not same before and aftere the dissolution.
For these case3s, we know the van’t Hoff’s factor(i)
for dissociation(like NaCl......Na+..& ...Cl-), i= 1+(n-1)*alpha
for association(like polymerisation,dmerisation,trimerisation,), i= 1+(1/n-1)*alpha
For more detail, please refer a text book.
And to solve the problem just apply simpl;e formulas given in text.