Question

Raoult’s law is not applicable in which of the following cases?
(A) $1M$ $NaCl$
(B) $1M$ Urea
(C) $1M$ Glucose
(D) $1M$ Sucrose

Answer 1

Hint: Solutes which dissociate or associate in a particular solution will not obey Raoult’s law. Raoult's law applies to solutions containing non-volatile solute only. Raoult’s law cannot be applied to concentrated solutions.

Complete step by step answer:
First, let us consider Raoult's law.
Raoult’s law states that the partial vapor pressure of any volatile constituent of a solution is equal to the vapor pressure of the pure constituent multiplied by the mole fraction of that constituent in solution.
Therefore we know that when a non-volatile solute is added to a solvent, the vapor pressure of the solvent is lowered.
Thus Raoult’s law can also be stated as the relative lowering of the vapor pressure of a solution containing a non-volatile solute is equal to the mole fraction of the solute in the solution. Ideal solutions obey Raoult’s law.

There are some limitations to Raoult’s law:
(i) Raoult’s law does not apply to the solutes which dissociate or associate in a particular solution.
(ii) Raoult’s law is only applicable to solutions containing non-volatile solute.
(iii) Raoult’s law does not apply to the concentrated solutions. It applies only to very dilute solutions.

Now, let us consider the options given in the question. Among these, we have to find out which one does not obey Raoult’s law. So we have to compare the limitations of Raoult’s law with these examples.
Urea, $NaCl$, glucose and sucrose are examples of non-volatile solute. The substances that cannot be vaporized easily are known as non-volatile solutes. So $NaCl$, urea, glucose and sucrose are non-volatile solutes.
We know urea, sucrose and glucose are non-electrolytes and $NaCl$ is an electrolyte.
Raoult’s law applies to solutes which do not change their nature when they dissolve. That is they must not ionize or associate.
If we put $NaCl$ in a solution, it dissociates as follows:
$N{a^ + }C{l^ - }_{(s)} \to N{a^ + }_{(aq)} + C{l^ - }_{(aq)}$.
> That is a mole of solid sodium salt dissociated into $2$ moles of ions in solution. Thus in the case of $NaCl$ Raoult’s law is not applicable since $NaCl$ dissociates in solution.
> Sucrose, glucose and urea are non-electrolytes which do not change their nature when dissolved in a solution. They will not ionize in solution. So they obey Raoult’s law.

Therefore the answer is an option (A) $NaCl$.

Note: Raoult’s law cannot be applied in the cases where the total number of particles of solute changes in solution due to association or dissociation. When a substance is dissolved in a solution if it ionizes to its constituents ions, it is dissociation. If we put a substance in solution and if it combines with the other substance in the solution and thus a molecule formation or a dimer formation takes place. It is an association. Both association and dissociation change the nature of the solute and in that case, Raoult’s law is not applicable.