Two elements A and B form compounds havi | 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 2, Question 21:

Two elements A and B form compounds having formula AB2 and AB4. When dissolved in 20 g of benzene (C6H6), 1 g of AB2 lowers the freezing point by 2.3 Kwhereas 1.0 g of AB4 lowers it by 1.3 K. The molar depression constant for benzene is 5.1 Kkg mol-1. Calculate atomic masses of A and B.

. 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 21:

Two elements A and B form compounds having formula AB2 and AB4. When dissolved in 20 g of benzene (C6H6), 1 g of AB2 lowers the freezing point by 2.3 Kwhereas 1.0 g of AB4 lowers it by 1.3 K. The molar depression constant for benzene is 5.1 Kkg mol-1. Calculate atomic masses of A and B.

Answer:

As We know that:

MB = (Kf x wB x 1000) / (wA X ΔTf)

Now ΔTf = 2.3 , wB = 1.0 , wA = 20, KF = 5.1 (given)

PUTTING THE VALUES IN THE EQUATION

MB = (5.1 x 1 x 1000) / (20 x 2.3) = 110.87 g/mol

Therefore MAB2 = 110.9

For AB4 compound

ΔTf = 1.3 , wb = 1 ,wa = 20

MB = (5.1 X 1 X 1000) / (20 X 1.3) = 196 g/mol

Therefore MAB = 196

Let x be the atomic mass of A & y be the atomic mass of B,

THEN MAB2 = x + 2y = 110.9 ----------------------------------(1)

And MAB = x + 4y = 196        ----------------------------------(2)

Subtracting 2 from 1 ,we get

2y  = 196-110.9

y   = 85.1 / 2 

y = 42.6

Putting the value of y in 1 we get

x = 110.9 - 2 x 42.6

x = 25.7

Therefore atomic mass of A = 25.7 u Atomic mass of B = 42.6 u.


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Comments

  • Baba Kareem Das
  • Mar 11, 2018

I Like this solutions


  • Satvir singh
  • May 12, 2017

In this question we are calculating the molecular mass of AB2 and AB4 molecule separately,therefore in case molecular mass of AB2 is taken and in other case molecular mass of AB4 is taken


  • Pinky
  • May 11, 2017

in molality we take the molecular mass of solution in denominator but here there is only molecular mass of compound AB2.Why?


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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 2 , Question 21: Two elements A and B form compounds having formula AB2 and AB4. When dissolved in 20 g of benzene (C....

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