Question

Why osmotic pressure method is preferred for determinig molar masses of macromolecules such as proteins?

Answer 1

There are many reason 
1. osmotic pressure can be measured at room temperature whereas other colligative properties need either high temp or low temp conditions. 
2. most of the macromolecules get denatured in high temp this can be avoided . 
3. Values of other colligative properties is very small that a small error can account to large difference.