The number of moles of solute present in one litre of solution is called molarity. It is represented by M . Molarity = Number of moles of solute Volume of solution in Litre = n V Molarity = weight of the solute Gram molecular weight of solute × 1 Volume of solution ( litres ) NOTE: Molarity is the most common way of expressing the concentration of a solution in the laboratory. However, it has one disadvantage. It changes with temperature because the volume of a solution alters due to expansion and contraction of the liquid with temperature.

Molality is the number of moles of solute present in one kilogram of a solvent. It is represented by m . It does not change with temperature because the mass of the solvent does not change so due to this reason it is a better way of determining the concentrations of solutions. m = Number of moles of solute Mass of the solvent in kg

What is an ideal solution?

Ideal solution is a homogeneous mixture of substances that has physical properties linearly related to the properties of the pure components. The classic statement of this condition is Raoult’s law, which is valid for many highly dilute solutions and for a limited class of concentrated solutions, namely, those in which the interactions between the molecules of solute and solvent are the same as those between the molecules of each substance by itself. Solutions of benzene and toluene, which have very similar molecular structures, are ideal: any mixture of the two has a volume equal to the sum of the volumes of the separate components, and the mixing process occurs without absorption or evolution of heat.

Vapour pressure of water at 293 K is 17 . 535 mm Hg . Calculate the vapour pressure of water at 293 K when 25 g of glucose is dissolved in 450 g of water.

Given:- P 0 = 17 . 535 mm Molar mass of glucose ( C 6 H 12 O 6 ) = 6 × 12 + 12 × 1 + 6 × 16 = 180 g mol - 1 Molar mass of water = 18 g mol - 1 Raoult ' s law p 0 - p s p 0 = n 2 n 1 + n 2 = n 2 n 1 = w 2 / M 2 w 1 / M 1 1 - p s p 0 = 25 / 180 450 / 18 = 25 × 18 180 × 450 1 - 450 81000 = p s p 0 p s = 0 . 9945 × 17 . 535 = 17 . 44 mm Hg

Raoult's law: According to this law, the partial pressure of any volatile constituents of a solution at a constant temperature is equal to the vapour pressure of pure constituents multiplied by the mole fraction of that constituent in the solution. For liquid-liquid solution: Let a mixture (solution) be prepared by mixing n A moles of liquid A and n B moles of li quid B . Let P A and P B be the partial pressures of two constituents A and B in solution and P A 0 and P B 0 the vapour pressures in pure state respectively. Thus, according Raoult's law, P A = X A P A 0 and P B = X B P B 0 .