Solutions

Concentration terms Solutions are the homogeneous mixtures of two or more than two components. The component that is present in large quantity is called solvent and the one present in small quantity is called solute. Solution can be classified as solid solutions, liquid solutions and gas solutions. Concentration of a solution is the amount of solute dissolved in a known amount of solution. The relative amount of solute in binary solution can be expressed by any of the terms, namely, Mass percentage, volume percentage, parts per million (ppm), Molarity (M), Molality (m), Normality (n) and Mole fraction. The maximum amount of substance that can be dissolved in a specific amount of solvent is known as solubility. Solubility of solid in liquid depends on the nature of the solvent, nature of the solute, temperature. Pressure does not have significant effect on the solubility of solids in liquids. Solubility of a gas in a liquid is defined as the volume of the gas (in cc.) that can dissolve in unit volume of the liquid to form the saturated solution at a given temperature. It depends on the nature of the gas and solvent, temperature and pressure. It increases with the increase in pressure and decreases with the increase in temperature. According to Henry’s law, the partial pressure of gas in vapour phase (p) is proportional to the mole fraction of gas () in the solution. There are various applications of Henry’s law such as in aerated drinks and in scuba diving. Raoult’s law Vapour pressure of a liquid is the pressure exerted by the vapour in equilibrium with the liquid. Addition of a non–volatile solute to a volatile solvent decreases its vapour pressure. This is called the lowering of vapour pressure. French Chemist, Raoult, gave the quantitative relationship between the lowering in vapour pressure and the concentration of solution. This quantitative relationship is known as Raoult’s law. For a solution of volatile liquids Raoult’s law states that the partial pressure of each component in the solution is directly proportional to its mole fraction. If the solution contains non-volatile solute, then according to Raoult’s law, the vapour pressure of a solution containing non–volatile solute is actually the vapour pressure of the solvent in the solution. A solution is a homogenous mixture of two or more chemically non–reacting species composed of only one phase. Solutions can be ideal or non–ideal depending on whether they obey the Raoult’s law or not. Non-ideal solutions are further classified as non-ideal solutions showing positive deviation from Raoult’s law and non-ideal solutions showing negative deviation from Raoult’s law. The solutions of completely miscible liquids, which boil at constant temperature and distil without any change in composition, are called constant boiling mixtures or azeotropic mixtures or simply azeotropes (boil unchanged). Colligative properties The properties which depend upon the number of solute particles present in a definite amount of solvent are known as colligative properties. Colligative properties do not depend upon the nature of the solute. The important colligative properties are relative lowering of vapour pressure, elevation in boiling point, depression in freezing point and osmotic pressure. The relative lowering in vapour pressure for a binary solution containing a non–volatile solute and a volatile solvent is equal to the mole fraction of the solute. The addition of a non – volatile solute in a solvent leads to the lowering of its vapour pressure. The solution formed has to be heated to a higher temperature so that its vapour pressure becomes equal to the atmospheric pressure. Hence, the boiling point of the solution is higher than that of the pure solvent. The difference in the boiling points of the solution and the pure solvent is called elevation in boiling point. Elevation in boiling point is directly proportional to the molality of the solution or elevation in boiling point is equal to the molality into molal elevation constant. Depression in freezing point is directly proportional to the molality of solution or depression in freezing point is equal to the product of molality and molal depression constant. Adding solute in a solvent leads to the lowering of the vapour pressure. The vapour pressure temperature curve for the solution lies below the curve for pure solvent. The spontaneous flow of the solvent through a semi permeable membrane (SPM) from pure solvent to a solution (i.e., from dilute solution to concentrated solution) is called osmosis. The excess pressure which must be exerted on solution side, in order to prevent the flow of solvent molecules towards the solution side, when both are separated by semi permeable membrane, is called osmotic pressure. On the basis of osmotic pressure, solutions are divided into isotonic, hypertonic and hypotonic. In reverse osmosis, pressure larger than osmotic pressure is applied on the solution side. As a result, the solvent from the solution starts flowing towards the pure solvent through a semi permeable membrane. van’t Hoff factor When the experimentally observed molecular mass of a substance, determined on the basis of a colligative property, is found to be different from the normal value as expected from its chemical formula, the observed molecular mass is referred to as abnormal molecular mass. This abnormal molecular mass is caused due to the dissociation or association of solute molecules. Certain solutes, when dissolved in a solvent (usually non–polar), undergo association to give dimer, trimer, tetramer, etc, forming bigger molecules. In such cases, the number of the solute particles decreases. The electrolytic substances such as NaCl, KCl, BaCl2 etc. dissociate into ions when dissolved in water. In such cases, the number of the solute particles increases. van’t Hoff factor is used to overcome the difficulties in determining the molecular mass of the solute, which undergoes dissociation or association in solution. van’t Hoff proposed a factor ‘i’, which is known as van’t Hoff factor. van’t Hoff factor is the ratio of the observed (experimental) value of a colligative property to the normal (calculated) value of the same property. It is used to express the extent of association and dissociation of a solute in a solution. When solute undergoes association in solution, ‘i’ is less than 1. When solute undergoes dissociation in solution, ‘i’ is greater than 1. van’t Hoff factor can be used to calculate the degree of dissociation and degree of association.

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