SELECT * FROM question_mgmt as q WHERE id=1580 AND status=1 SELECT id,question_no,question,chapter FROM question_mgmt as q WHERE courseId=3 AND subId=9 AND chapterId=57 and ex_no='2' AND status=1 ORDER BY CAST(question_no AS UNSIGNED) CBSE Free NCERT Solution of 12th chemistry Solutions two elements a and b form compounds having formula

Question:

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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