Two elements A & B form compounds having molecular formulae AB2 and AB4. When dissolved in 20.0 g of benzene 1.00 g of AB2 lowers FP. by 2.3° C whereas 1.00g  of AB4 lowers FP. by 1.3°0 C. The molal depression constant for benzene in 1000 g is 5.1. Calculate the sum of atomic masses of A and B.

Let atomic masses of A and B be a and b amu respectively∴ Molar mass of AB2=(a+2 b)gmol-1 and Molar mass of AB4=(a+4 b)gmol-1For compound AB2 ΔTb=Kb×WB×1000/WA×MB2.3=5.1×1×1000/20.0×(a+2 b)….IFor compound AB41.3=5.1×1×1000/20.0×(a+4 b)… ...II  Solving (I) and (II),a=25.49 b=42.64 Sum of a and b=25.49+42.64=68.13