Question

$0.1M$ urea solution shows less depression in freezing point than $0.1M$ $MgC{{l}_{2}}$ solution. Explain.

Answer 1

You should know that; freezing point is a colligative property observed in solutions that results from the introduction of solute molecules to solvent. Find out the total concentration of the particles of the following compounds given and figure out which one has the what concentration.

Complete step by step solution:
Given that,
-$0.1M$ urea solution shows less depression in freezing point than $0.1M$ $MgC{{l}_{2}}$ solution.
-So, we know that the chemical formula of urea is $N{{H}_{2}}CON{{H}_{2}}$.
And, here as per the given question,
-The concentration of the urea solution, let us consider it as ${{M}_{1}}$ is given as $0.1M$.
While, the concentration of the magnesium chloride solution i.e. $MgC{{l}_{2}}$, consider it as ${{M}_{2}}$ is given as $0.1M$.
-Now, let’s see what happens to these compounds when they are introduced to an aqueous solution.
-In case of urea solution, as it is not an electrolyte it will not dissociate in the aqueous solution. So, the total concentration of its particle will remain same i.e. $0.1M$.
-While in case of magnesium chloride solution, it is an electrolyte. When this solution is treated with an aqueous solution, it dissociates to give three ions i.e. one magnesium ion and two chlorine ions. The reaction of the dissociation of magnesium chloride solution is given below:
$MgC{{l}_{2}}\xrightarrow{Aqueous}M{{g}^{+}}+2C{{l}^{-}}$
-Therefore, the total concentration of the particles of magnesium chloride will be $(0.1+2\times 0.1)M=0.3M$.
-So, we can see that the number of particles of solute in $MgC{{l}_{2}}$ solution is more than in urea solution. Thus, the total concentration of the particles of $MgC{{l}_{2}}$ solution is more than that of urea solution. Due to this, depression in freezing point of $0.1M$ urea is less than that of $0.1M$ $MgC{{l}_{2}}$ solution because of depression of freezing point is a colligative property and it depends upon the number of the solute particles.

Note: It is important to note that, the freezing point of all solutions are always lower than that of the solvent and it is generally directly proportional to the molality of the solute that is present in a solution.