Which of the following is appropriate for the solution made by mixing acetone and carbon disulphide?

Negative deviation from Raoult's law

State and prove Raoult's law for non-volatile solute in volatile solvent. Also give any two limitations of Raoult's law.

Raoult's law states that the vapour pressure of a solution having a non-volatile solute is equal to the vapour pressure of the pure solvent at that temperature multiplied by its mole fraction. p T = p B 0 . χ B In this equation, p B 0 is the vapour pressure of the pure solvent at that same temperature, p T is the total pressure of the solution, χ B is the mole fraction of the solvent. For a solution of a non-volatile solute (A) and volatile solvent (B) total pressure is sum of the partial pressure of the two as: p T = p A 0 . χ A + p B 0 . χ B Since A is non-volatile, its vapour pressure p A 0 will be zero. Hence, p T = p B 0 . χ B Limitations of Raoult's law: 1. The intermolecular forces between the solvent and solute components should be similar to those between individual molecules. 2. The gaseous phase are assumed to behave ideal where ideal gas law can be applied.

Two elements A and B form compounds having the formulae AB 2 and AB 4 . When dissolved in 20 g of benzene ( C 6 H 6 ), 1 g of AB 2 lowers the freezing point by 2 . 3 K , where as 1 g of AB 4 lowers it by 1 . 3 K . The molar depression constant for benzene is 5 . 1 K kg mol - 1 . Calculate atomic masses of A and B .

By using the depression in freezing point formula, M 2 = 1000 × k f × w 2 w 1 × ∆ T f M AB 2 = 1000 × 5 . 1 × 1 20 × 2 . 3 = 110 . 87 g mol - 1 M AB 4 = 1000 × 5 . 1 × 1 20 × 1 . 3 = 196 . 15 g mol - 1 Let atomic masses of A and B are p and q . molar mass of AB 2 = p + 2 q = 110 . 87 g mol - 1 1 molar mass of AB 4 = p + 4 q = 196 . 15 gmol - 1 2 By subtracting equations (1) and (2) 2 q = 85 . 28 , q = 42 . 64 Put value of q in equation (1) p = 110 . 87 - 85 . 28 p = 25 . 59 Atomic mass of A = 25 . 59 g mol - 1 Atomic mass of B = 42 . 64 g mol - 1

Write van't Hoff's laws of osmotic pressure, give its mathematical formula and derive a formula to find osmotic pressure.

Van't Hoff's first law (Van't Hoff-Boyle's law): The osmotic pressure ( π ) of a solution is directly proportional to its molar concentration ( C ) at a constant temperature. Mathematically, π ∝ C , or π = k 1 C -----(i), where k 1 is a constant. Van't Hoff's second law (van't Hoff-Charles law): The osmotic pressure of a solution of given concentration varies directly as the temperature in Kelvin ( K ). Mathematically, π ∝ T , or π = k 2 T ---------(ii), where k 2 is a constant. Van't Hoff's third law (Combined solution equation): Equimolecular quantities of different solutes dissolved in such volumes of the solvent as to give the same volume of the solution have the same osmotic pressure at the same temperature. By combining equations (i) and (ii) we can write, π = R C T , this equation is called van't Hoff's equation. R is called the solution constant, and its value is same as that of gas constant. If W g of a solute of molecular mass M is dissolved in V dm 3 of the solution, then the molar concentration, C = W MV , therefore, van't Hoff's equation π = R C T becomes, π = RTW MV = nRT V . Hence, the osmotic pressure, π = nRT V , where n is the number of moles of the solute.

E M B I B E

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR CHEMISTRY>Chapter -1 - Solutions>Gujarat Board-2020>Q 9

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12th Gujarat Board

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Which of the following is an example of a solid solution in which solute is a gas?

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