Question

Vapour pressure of pure ‘a' is 70 mm of hg at 25

c. it forms an ideal solution with ‘b' in which mole fraction of a is 0.8. lithe vapour pressure of the solution is 84 mm of hg at 25°c. the vapour pressure of pure ‘b' at 25°c is

Answer 1

Answer: The vapour pressure of pure ‘b' at 25°c is

Explanation: According to Raoult's law, the vapor pressure of a component at a given temperature is equal to the mole fraction of that component multiplied by the vapor pressure of that component in the pure state.

$p_a=x_ap_a^0$ and $p_b=x_bP_b^0$

where, x = mole fraction

$p^0$ = pressure in the pure state

According to Dalton's law, the total pressure is the sum of individual pressures.

$p_{total}=p_a+p_b$

$p_{total}=x_ap_a^0+x_bP_b^0$

given $x_a=0.8$, $x_b=0.2$

 as  $x_a+x_b=1$

$p_a^0=70 mmHg$

$p_b^0=? mmHg$

$p_{total}=84 mmHg$

$84=0.8\times 70+0.2\times p_b^0$

$p_b^0=140mmHg$