Question

For which among the following equimolar aqueous solutions Van't Hoff factor has the lowest value?
a.) Aluminium Chloride
b.) Potassium Sulphate
c.) Ammonium Chloride
d.) Urea

Answer 1

Hint: The Van’t Hoff factor is a measure of the effect of a solute on colligative properties, such as osmotic pressure, relative lowering in vapour pressure, boiling point elevation and freezing point depression. We need to find the Van’t Hoff factor of each of the mentioned options, to determine the one having the lowest.

Complete step-by-step answer:
In the first place, we have Aluminium Chloride. The chemical formula of Aluminium Chloride is $\text{AlC}{{\text{l}}_{\text{3}}}$. We know Van’t Hoff factor is defined as i.
So in the case of $\text{AlC}{{\text{l}}_{\text{3}}}$, the value of i is 4.
The equation is given below:
$\text{AlC}{{\text{l}}_{\text{3}}}\text{ }\xrightarrow{{}}\text{ A}{{\text{l}}^{\text{3+}}}\text{ + 3C}{{\text{l}}^{-}}$
Next we have Potassium Sulphate. The chemical formula of Potassium Sulphate is ${{\text{K}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$.
So in case of ${{\text{K}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}$, the value of i is 3.
The equation is given below:
${{\text{K}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{ }\xrightarrow{{}}\text{ 2}{{\text{K}}^{\text{+}}}\text{ + S}{{\text{O}}_{\text{4}}}^{\text{2}-}$
Now, for Ammonium Chloride, we have the formula as $\text{N}{{\text{H}}_{\text{4}}}\text{Cl}$.
So in the case of $\text{N}{{\text{H}}_{\text{4}}}\text{Cl}$, the value of i is 2.
The equation is given below:
$\text{N}{{\text{H}}_{\text{4}}}\text{Cl }\xrightarrow{{}}\text{ N}{{\text{H}}_{\text{4}}}^{\text{+}}\text{ + C}{{\text{l}}^{-}}$
Lastly, we have Urea. We know that the aqueous solution of Urea Van't Hoff factor has the lowest value. Urea is non electrolyte and does not dissociate an aqueous solution. Hence the value of i is 1.

So we can see that Urea has the lowest Van’t Hoff value. So the correct answer is Option D.

Note: To measure the Van't Hoff factor we need to find the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass.
For most non electrolytes dissolved in water the Van't Hoff factor is essentially 1. For most ionic compounds dissolved in water, the Van't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. This is true for ideal solutions only, as occasionally ion pairing occurs in solution.