Assertion: lowering of vapour pressure is directly proportional to osmotic pressure of the soln.
Reason: osmotic pressure isa colligative property
Tell what is right or wrong
Answers
Explanation:
Both assertion and reason are correct but the reason is not the correct explanation of assertion.
The relationship between the lowering of vapour pressure and osmotic pressure can be derived as follows:
Van't Hoff equation for dilute solutions is
π=
V
n
RT.....(i)
In case of a dilute solution. The volume of solution can be taken as equal to that of the solvent. If N is the number of moles of the solvent of molecular weight M and density ρ, the volume V is given by
V=
ρ
NM
.......(ii)
or
N
n
=
ρ.R.T.
πM
....(iii)
From Raoult's law,
P
∘
P
∘
−P
=
N
n
.......(iv)
∴
P
∘
P
∘
−P
=
ρRT
πM
[From (iii) and (iv)]
or (P
∘
−P)=
ρRT
πM
×P
∘
The factor
ρRT
MP
∘
is constant at constant temperature.
∴(P
∘
−P)∝π
or lowering of V.P.∝ osmotic pressure
Thus assertion is correct.
Osmotic pressure is a colligative property is correct