Saved Bookmarks
| 1. |
Vapour pressure of benzene is 200 mm of Hg. When 2 gram of a non-volatilesolute dissolved in 78 gram benzene, benzene has vapour pressure of 195 mm of Hg. Calculate the molar mass of the solute. [Molar mass of benzene is 78 g/mol^(-1)] |
|
Answer» Solution :(a) `M_(2) = (W_(2) xx M_(1) xx P_(1)^(0))/((P_(1)^(0) - P_(a)) xx W_(1))` `M_(2) = (2 xx 78 xx 200)/((200 - 195) xx 78)` `M_(2) = 80 g//mol` (b) Binary liquid mixtures having the same COMPOSITION in liquid and vapour phase and boil at a constant temperature. or Constant boiling point liquid mixture [Any suitable example for solution minimum boiling azeotrope.] benzene and ACETONE, n-Hexane and ethanol, water and ethanol, acetone and `CS_(2), C Cl_(4)` and `CHCl_(3), C Cl_(4)` and Toluene, Acetone and ethanol. (a) Given * Vapour pressure of pure benzene `(P_(0))` = 200 mm of Hg. * Mass of non-volatile SOLUTE `(W_(B) = 2g` * Mass of benzene as solvent `(W_(A)) = 78 g` * Molar mass of benzene `(M_(A)) = 78 g mol^(-1)`. * Vapour pressure of solution (P) = 195 mm of Hg `because` No. of moles (n) `= ("W(mass)")/("(M(Molar mass)")` `:.` Moles of benzene (n) `= (W)/(M) = (78)/(78) = 1` and Moles of non-volatile solute `(n_(B)) = (W_(B))/(M_(B)) = (2)/(M_(B))` To find : Value of `M_(B)`. also `(DELTA p)/(p^(0)) = (n_(B))/(n_(A))` `(p^(0) - p)/(p^(0)) = (n_(B))/(n_(A))` `(Delta p)/(p^(0)) = (200 - 195)/(200) = ((2)/(M_(B)))/((1)/(1))` `:. (5)/(200) = (2)/(M_(B))` or `M_(B) = (2 xx 200)/(5) = 80` Molar mass of solute `= 80 g mol^(-1)`. (b) Azotropes : it is a mixture of two liquids which has a constant boiling point and composition through out distillation. It is of two types : The minimum boiling azeotrope and the maximum boiling azeotrope. A solution that shows (+) ve DEVIATION from Raoult.s law is called minimum boiling zeotrope while a solution that shows (-) ve deviation from Raoult.s law is called maximum boiling azeotrope. Mixture of ethanol and water, in which ethanol is approximately 95% by volume is an example of minimum boiling azeotrope. |
|