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XII 02 Solutions VIII
Depression in freezing point Freezing point of a substance in defined as the temperature at which the vapour pressure of its liquid is equal to the vapour pressure of the corresponding solid. The difference between the freezing points of the pure solvent and its solultion g points of the pure solvent and ltion is called depression of freezing point.
From Raoult's law of dilute solutlons: po-pWB MA 0 MR W For the pure solvents Po and MA are constants. Therefore W B MR.WA W B MRWA MR.WA AT- K.WB MR.WA When WB - 1(one mole of solute)and WA 1g
K can be calculated by the following formula: 0.002 TE2 T, freezing point of the solvent. Lf latent heat of fusion
Solvent Freezing point (C) Molal elevation constant 1. Water 2. Ethyl alcohol 3. Chloroform 1.86 1.99 9.70 29.80 5.12 39.70 0.0 114.6 63.5 22.8 5. Benzene 6. Camphor 5.5 179.0 If Kf,wa,AT and Wg are known, molecular mass of the non-volatile solute can be determined
Antifreeze solutions: Antifreeze solutions are useful under these conditions when the vehicles are used in the regions ofsub zero temperatures so that water does not freeze in radiations. Such solutions are made by dissolving ethylene glycol in water. Freezing point can be lowered to disired extent by varying the concentration ofethylene glycol. Glycerol can also be used as antifreeze. Freezing mixture: It is a mixture ofice and common salt. It is used in making of ice-cream and in the laboratories to create low temperature. The temperature can be lowered to -33 C