Why determination of osmotic pressure is preffered for finding molecular mass of macro-molecules?

Explanation

According to van.t Hoff theory of dilute solutions
`pi = CRT `
Where `pi` is osmotic pressure, C is molar concentration of solution and T is the temperature on Kelvin scale .
R is a consant, which is same as gas constant `(0 . 08205 L atm K^(-1) mol^(-1))`
Now `C = ( n)/(V) = ("No. of moles of solute")/(" Volume of solution in litres")`
`pi = (n)/( V) RT `
or `pi V = n RT `
If w gms of solute is dissolved in V litres of solution and M is molecule mass of solute then
`pi V = (w)/( M) RT `
`M= (w RT)/( pi V)`
As the molecular masses of macro- molecules are of the order of `10^(6)` a.m.u. the molality of the solution taken is very low. For such solutions. other methods like elevation in boiling point, depression in freezing point etc. cannot be used because change measured in these properties are very small and thus cannot be accurately , measured. On the other hand, osmotic pressure for such solutions can be easily measured du to its much larger magnitude