Question
Explain abnormal molar Tasses. Also explain association and dissociation of Solute.
✓
Answer
$\rightarrow$ Ionic compounds when dissolved in water dissociate into cations and anions.
$\rightarrow$ For example, if we dissolve one mole of $KCI (74.5 \ g)$ in water, we expect one mole each of $K^+$ and $Cl^-$ions to be released in the solution. If this happens, there would be two moles of particles in the solution.
$\rightarrow$ one mole of $KCl$ in one $\ kg$ of water would be expected to increase the boiling point by $2 \times 0.52 \ K = 1.04 \ Κ$
$\rightarrow$ If we did not know about the degree of dissociation, we could be led to conclude that the mass of $2 \ mol$ particles is $74.5 \ g$ and the mass of one mole of $KCl$ would be $37.25 \ g.$
$\rightarrow$ When there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value.
$\rightarrow$ Molecules of ethanoic acid (acetic acid) dimerise in benzene due to hydrogen bonding. This normally happens in solvents of low dielectric constant. In this case the number of particles is reduced due to dimerisation.
$\rightarrow$ It can be undoubtedly stated here that if all the molecules of ethanoic acid associate in benzene, then $\Delta T _{ b }$ or $\Delta T _{ f }$ for ethanoic acid will be half of the normal value.
$\rightarrow$ The molar mass calculated on the basis of this $\Delta T_b$ or $\Delta T_f$ will, therefore, be twice the expected value. Such a molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass.
$\rightarrow$ For example, if we dissolve one mole of $KCI (74.5 \ g)$ in water, we expect one mole each of $K^+$ and $Cl^-$ions to be released in the solution. If this happens, there would be two moles of particles in the solution.
$\rightarrow$ one mole of $KCl$ in one $\ kg$ of water would be expected to increase the boiling point by $2 \times 0.52 \ K = 1.04 \ Κ$
$\rightarrow$ If we did not know about the degree of dissociation, we could be led to conclude that the mass of $2 \ mol$ particles is $74.5 \ g$ and the mass of one mole of $KCl$ would be $37.25 \ g.$
$\rightarrow$ When there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value.
$\rightarrow$ Molecules of ethanoic acid (acetic acid) dimerise in benzene due to hydrogen bonding. This normally happens in solvents of low dielectric constant. In this case the number of particles is reduced due to dimerisation.
$\rightarrow$ It can be undoubtedly stated here that if all the molecules of ethanoic acid associate in benzene, then $\Delta T _{ b }$ or $\Delta T _{ f }$ for ethanoic acid will be half of the normal value.
$\rightarrow$ The molar mass calculated on the basis of this $\Delta T_b$ or $\Delta T_f$ will, therefore, be twice the expected value. Such a molar mass that is either lower or higher than the expected or normal value is called as abnormal molar mass.
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