Write a short note on abnormal molecular mass.
Answers
Answer:
Abnormal Molecular Masses. Molecular masses can be calculated by measuring any of the colligative properties. The relation between colligative properties and molecular mass of the solute is based on following assumptions. The solute neither undergoes dissociation nor association in solution.
Explanation:
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Answer:
Molar masses that are lower or higher than expected values when calculated (generally using colligative properties) are called abnormal molar masses.
Abnormal molar masses depends upon the total number of moles particles either after dissociation or association of solute molecules in solvent or solution.
Dissociation of Solutes:
There are many solutes, such as organic acids, salts and bases, when dissolved in water dissociate into ions.
Example – When one mole of NaCl is dissolved into water, it dissociates into Na + and Cl - . After dissociation, there would be one mole of sodium ions (Na+) and one mole of chloride ions (Cl- ) released in solution. Thus, we get two moles of particles in the solution.
Explanation:
After the dissociation of NaCl or KCl or any other solutes which breaks into ions into water, the boiling point of water increases.
And when molar mass is determined experimentally, it always comes lower than the expected value.
Association of Solutes:
There are many solutes, when dissolved, get associated into the solution, this decreases the number of moles into the solution. This increases the molar mass of the solution than expected value, when determined experimentally, using colligative properties methods.
Example –
When acetic acid (ethanoic acid) is dissolved into benzene, molecules of acetic acid gets associated i.e. dimerize due to hydrogen bondings and number of particles reduced. This happens generally in the solvents having low dielectric constants.
This increases the molar mass than expected value when determined experimentally