Question

Molality is preferred over Molarity, then why for determining molecular maas we prefer osmotic pressure method( it also uses molarity)?

Answer 1

Molar mass can be determined by osmotic pressure method.
According to law of osmotic pressure (Π):

Π α c ... (i)
Π α T ... (ii)
(c= concentration and T= absolute temperature)
Equation (i) and (ii) is analogous to Boyle’s and Charle’s law.
Combining these two we get
Π α cT

Or Π = K c T ...(iii)
K is proportionality constant
Ideal gas equation is P= n/V RT = cRT .... (iv)
Where n/V = c = molar concentration.

Equation (iii) and (iv) are identical (pointed out by van’t Hoff) and experimentally determined value of K = 0.082 litre-atmosphere/degree. So c in equation (iii) is molar concentration.
Thus concentration in osmotic pressure equation is in molar concentration.