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Chemistry Level 2

Two volatile liquids A A and B B are taken in two separate container P P and Q Q . 10 moles Liquid A A is poured in container P P and 5 moles of liquid B B is poured in container Q Q . Now in container P P , 5 moles of non-volatile solute (molecular formula MX2) is mixed whereas in container Q Q , 3 moles of another non -volatile solute Y(molecular formula M2X3)h is mixed.

Now the two solutions formed in P P and Q Q are mixed together mixed in third container. Now the total vapour pressure of mixture of A , B , X , Y A,B,X,Y is noted and reported. Find the reported Vapour pressure.

Given:-

  • Vapour pressure of A and B in pure state is 180 mm Hg and 270 mm Hg respectively.

  • solutions are ideal

  • non-volatile solutes ionize completely in A and B

  • All components of solutions are non-reacting.


The answer is 70.

Puneet Mangla by puneet mangla , 21, India

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2 solutions

Aniket Sanghi
Dec 14, 2016

Other way of understanding could be

Let us first mix A and B

Then pressure measured is

P = P A X A + P B X B = 210 m m o f H g P = {P}_{A} {X}_{A} + {P}_{B} {X}_{B} = 210 mm \quad of \quad Hg

Now you added solute

Effective moles of solute added = 5 × 3 for X and 3 × 5 for Y = 30 moles

Now , using lowering of vapour pressure formula

P ○ − P s P ○ = m o l e f r a c t i o n o f s o l u t e \frac {{P}_{○} - {P}_{s}}{{P}_{○}} = mole \quad fraction \quad of \quad solute

P s = 70 m m o f H g {P}_{s} = 70 mm \quad of \quad Hg

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Puneet Mangla
May 14, 2016

The problem is based on Raoult's Law .

In case of solution of two non volatile liquids the V.P of the component is expressed as V.P= XP, where X is mole fraction of solvent and P is its vapour pressure at pure state. same is in case when a non-volatile solute and solvent is mixed.

So in this problem both solution of solvent and solution of solute is present.

now,

for container P

moles of A=10

moles of solute= 3 x 5 ( as no of effective moles is 15)

for container Q

moles of B=5

moles of solute=15 (same reason)

now , when both are mixed,

total moles= 45

V.P= Xa x Pa + Xb x Pb

=70mm Hg

1 Helpful 0 Interesting 0 Brilliant 0 Confused

Why don't your questions get level ? Don't you give a level preference to your question before posting ?

Aniket Sanghi - 4 years, 6 months ago

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