9.
The degree of dissociation of Ca(NO), in a dilute aqueous
solution containing 14 g of the salt per 200 g of water at
100°C is 70%. If the vapour pressure of water is 760 mm of
Hg, what will be the vapour pressure of the solution.
(a) 846 mm of Hg (b) 746 mm of Hg
846
(c) a atm
(d) 782 mm of Hg
760
Answers
we need to find the moles of CA(no3)2
and water
to find vapour pressure=Po-P/Po=n/n+N
we will take one mole of Ca(NO3)2
degree of dissociation of Ca(NO3)2=70/100=0.7
Ionization of Ca(NO3)2 can be represent as=Ca(NO3)2=Ca2+ +2NO-3
At start 1 0 0
at equilibrium 1-0.7 0.7 2* 0.7
therefore, total number of moles in the solution at equilibrium
=(1-0.7)+0.7+2*0.7=2.4
numbers of moles when the solution contains 1gm of calcium nitrate instead of 1 mole of the salt
=2.4/164(164 is the mol.wt. of Cal. nitrate)
thus, number of moles of the solute in the solution containing 7 g of salt
n=2.4/164*7=0.102
no.of moles of water (N)=weight of water/ mol.wt. of water=100/18=5.55
applying raoult's law =Po-P/Po=n/n+n
760-p/760=0.102/0.102+5.55
760-p/760=0.0180
p=760-(760*0.0180)
=746.3 mm Hg.