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.

Question

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.

Infinity Learn · Accepted Answer

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 $$AB2$$ Δ$$Tb=Kb$$×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 $$b=25.49+42.64=68.13$$

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.

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