# 2.21    Two elements A and B form compounds having formula AB2 and AB4 . When dissolved in 20 g of benzene   of AB2 lowers the freezing point by  whereas of AB4 lowers it by . The molar depression constant for benzene is Calculate atomic masses of A and B.

In this question we will use the formula :

Firstly for compound AB2  :-

or

Similarly for compound AB4 :-

If we assume atomic weight of element A to be x and of element B to be y, then we have :-

x + 2y = 110.87                               -----------------   (i)

x + 4y = 196                                    -----------------   (ii)

Solving both the equations, we get :-

x  =  25.59  ;    y =  42.6

Hence atomic mass of element A is 25.59u and atomic mass of element B is 42.6u.

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