A solution of a anon volatile solute in water freezes at -30 degree C. The vapour pressure ofthe pure water at 298K is 23.51 mm Hg and Kf for water is 1.86 K kg/mol.
Calculate the vapour pressure o fthis solution at 298k
11ΔP/P° = n/(n+N)
For very dilute soln. n<<N
So, ΔP/P° = n/N
Usually this approximation does not bring about a considerable change in the final ans.
BUT....
Here ,
Ans. : 21.23489 mm (with the approximation)
and 18.22025 mm (without the approximation)
11ΔTf=Kf m
S
30=1.86 x m
m=30/1.86 = 16.12
AND WE KNOW THAT
XA/XB X 1000/MB=m
16.12=X/1-X X1000 /18
X=0.224
AND
p0-ps/p0=Xsolute(a non volatile solute is mentioned )
23.51-ps/23.51 =Xs
23.51-ps/23.51=0.224
ps =18.24