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.

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Solution

We know that,

Then,

= 110.87 g mol⁻¹

= 196.15 g mol⁻¹

Now, we have the molar masses of AB₂ and AB₄ as 110.87 g mol⁻¹ and 196.15 g mol⁻¹ respectively.

Let the atomic masses of A and B be x and y respectively.

Now, we can write:

Subtracting equation (i) from (ii), we have

2y = 85.28

⇒ y = 42.64

Putting the value of ‘y’ in equation (1), we have

x + 2 × 42.64 = 110.87

⇒ x = 25.59

Hence, the atomic masses of A and B are 25.59 u and 42.64 u respectively.

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QUESTION 2.21

Two elements A and B form compounds having formula $A B_{2}$ and $A B_{4}$ . When dissolved in 20 g of benzene $(C_{6} H_{6})$ , 1 g $A B_{2}$ lowers the freezing point by 2.3 K whereas 1.0 g of $A B_{4}$ lowers it by 1.3K. The molal depression constant for benzene is 5.1 K kg $m o l^{- 1}$ . Calculate atomic masses of A and B.

Two elements A and B form compounds of formula AB₂ and AB₄. When dissolved in 20.0g of benzene 1.0g of AB₂lowers freezing point by 2.3^°C whereas 1.0g of AB₄ lowers freezing point by 1.3^°C. The K_f for benzene. The atomic masses of A and B are