Question

Which of the following units can be used to represent solubility?
A. $g$
B. $mol$
C. $mol{{L}^{-1}}$
D. $mL{{s}^{-1}}$

Answer 1

A measure of the tendency of solute particles to dissolve in solvent (usually water) to form solution is known as solubility. In general, the solubility of solids in solvent increases with temperature while the solubility of gases decreases with increasing temperature.

Complete answer: The solubility can be expressed in three ways, which are as follows:
Representation of solubility in terms of gram equivalents:
When the solubility is defined in terms of mole equivalents or gram equivalents per litre solvent, then it is known as normality of the solution. It is majorly used in titration reactions or reactions involving acid-base chemistry. The expression of normality is as follows:
$N=\dfrac{\text{number of gram equivalents of solute}}{\text{Volume of solvent in litres}}$
And the unit of normality is ${{\text{g}}_{eq}}{{\text{L}}^{-1}}$.
Representation of solubility in terms of moles of solute:
When the solubility is defined in terms of the number of moles of solute per litres of a solution, then it is known as molarity of the solution. The expression of molarity is as follows:
$M=\dfrac{\text{number of moles of solute}}{\text{Volume of solution in litres}}$
And the unit of molarity is $\text{mol }{{\text{L}}^{-1}}$.
Representation of solubility in terms of mass of solvent.
When the solubility is defined in terms of amount of substance i.e., number of moles of solute dissolved per kilogram of solvent, then it is known as molality of the solution. The expression of molality is as follows:
$m=\dfrac{\text{number of moles of solute}}{\text{mass of solvent in kg}}$
And the unit of molality is $\text{mol k}{{\text{g}}^{-1}}$.
Hence among given options, the unit which is used to represent solubility is $\text{mol }{{\text{L}}^{-1}}$.
Thus, option (C) is the correct answer.

Note:
It is important to note that there is a very minute difference between the terms solubility and concentration. Solubility refers to the highest concentration or maximum amount of solute which can be dissolved in the given amount of solvent whereas concentration is the amount of solute dissolved in unit quantity of the solution.