The vapour pressure of pure liquids A an | Class 12 Chemistry Chapter Solutions, Solutions NCERT Solutions

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Academic session 2026-27, Chapter 1 in the current curriculum.

Welcome to the NCERT Solutions for Class 12 Chemistry - Chapter Solutions. This page offers a step-by-step solution to the specific question from Exercise 1, Question 8:

The vapour pressure of pure liquids A and B are 450 and 700 mm Hg respectively, at 350 K. Find out the composition of the liquid mixture if total vapour pressure is 600 mm Hg. Also find the composition of the vapour phase.

. With detailed answers and explanations for each chapter, students can strengthen their understanding and prepare confidently for exams. Ideal for CBSE and other board students, this resource will simplify your study experience.

Question 8:

The vapour pressure of pure liquids A and B are 450 and 700 mm Hg respectively, at 350 K. Find out the composition of the liquid mixture if total vapour pressure is 600 mm Hg. Also find the composition of the vapour phase.

Answer:

It is given that:

PAo = 450 mm of Hg

PBo = 700 mm of Hg

ptotal = 600 mm of Hg

From Raoult's law, we have:

ptotal = PA + PB

 

Therefore, xB = 1 - xA

= 1 - 0.4

= 0.6

 

Now,  PA = PAo xA

= 450 × 0.4

= 180 mm of Hg

and PB = PBo xB

= 700 × 0.6

= 420 mm of Hg

Now, in the vapour phase: Mole fraction of liquid A =  PA / (PA + PB )

=180 / (180+420)

= 180/600

= 0.30

And, mole fraction of liquid B = 1 - 0.30

= 0.70


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Comments

  • Bhavesh
  • Oct 14, 2019

Aneesha ,what you have a doubt is another representation of the same formula😂. p(total)=p(a)+p(b) which is p°(a)x(a)+p°(b)x(b) But we also know that sum of mole fractions is 1 x(b)=1-x(1) Deducing it we get the simpler form


  • Lia
  • Jun 18, 2019

Similar doubt as that of anisha's


  • Tayaba
  • May 13, 2019

What does it mean by composition ? Either it mean mole fraction or vapour pressure ?


  • rahul
  • Apr 24, 2019

the query of aneesha is a poor one... Ask something good baby...


  • Shivam
  • Jun 12, 2018

It ask for composition. In your answer you give partial vp so it means composition means partial vapour pressure.


  • Aneesha
  • Mar 25, 2018

Hello. I have a small query. By using the formula P(total)=P(A)+[P(B)-P(A)]X(B), we get the value of X(B) as 0.42 and X(a) as 0.58. If done in this way, we end up multiplying 450 with 0.58 and 700 with 0.42. Why then am i ending up in the reverse order ? (the formula is given in the NCERT textbook-in terms of 1 and 2 instead of A and B)


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Welcome to the NCERT Solutions for Class 12 Chemistry - Chapter . This page offers a step-by-step solution to the specific question from Excercise 1 , Question 8: The vapour pressure of pure liquids A and B are 450 and 700 mm Hg respectively, at 350 K. Find out t....

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