CBSE Class 12-science Answered

Properties of solutions that depend on the number of molecules present and not on the kind of molecules are called colligative properties. These properties include boiling point elevation, freezing point depression, and osmotic pressure.  Colligative properties are useful for determining molecular weights of unknown samples, because they depend only upon the amount of sample dissolved in a known solvent, and not on the identity of the sample. For example: if you have to determine the molecular weight of an unknown substance from its freezing point depression in tertiary-butanol, or t-butanol. Then we will first determine the freezing point of pure t- butanol, which we will use as a solvent, and also the freezing points of two solutions of an unknown mixed with t-butanol. From the resulting change in temperature () that we observe, we will be able to calculate the molar mass of the unknown.