What is Molar Solubility?

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Solubility of gases in Water

Molar solubility (M) is the number of moles of a substance (the solute) that can be dissolved per liter of solution before the solution becomes saturated. The mole is a unit of measurement for the amount of substance or chemical amount. Molar solubility can be calculated from a substance's Solubility Product constant (Ksp) and stoichiometry. The units are mol/L, sometimes written as M. Molar solubility (M), in other words, is a measure of the ability of a compound, called a solute, to dissolve in a specific substance, called a solvent. Particularly, it is the maximum number of moles of a solute that are able to dissolve in one liter of solvent, so molar solubility is measures as moles/L. The solution is considered to be saturated, when the number of moles dissolved in a liter of solvent is equal to the molar solubility, which means, it could not dissolve more solute. In General Chemistry subject, Molar solubility problems is very essential.


What is the importance of Solubility Product constant (Ksp)?

The value of M depends on different variables. One important factor is the solubility product constant, which is designated Ksp. This value is constant for each compound that describes how easily a substance dissolves. Moreover, the ratio of products and reactants in the chemical reaction is required to calculate the value of M. This is referred to as the stoichiometry of the reaction.


What is the Definition of Molar solubility?

The definition of molar solubility is stated as “The molar solubility of a slightly soluble salt in water is the formal concentration of a salt dissolved when added to pure water and allowed to come to equilibrium”.


How to determine Molar Solubility?

There are some steps involved in the calculation of molar solubility.

  • Step 1: With the reactant on the left and products on the right, the chemical equation is balanced for the dissociation of the substance. In order to achieve this, one should make sure that there are equal numbers of each atom on both the product and reactant side of the chemical reaction.
  • Step 2: By checking whether the ionic compounds are solid, aqueous, liquid or gas, solids and liquids can be discarded. This is because; the solubility of a solid or liquid is not going to change.
  • Step 3: The dissociation of silver chloride is taken as an example. The balanced chemical equation of the dissociation of a molecule of silver chloride with equal numbers of each atom on the product and reactant sides is given as (AgCl) into silver (Ag) and chlorine (Cl) atoms: AgCl (s) → Ag+ (aq) + Cl - (aq). The physical state of the atoms and molecules is designated using ‘s’ for solid and ‘aq’ for aqueous or dissolved in water.
  • Step 4: In order to find out M, one should know the Ksp value for the solute, or the starting compound that is being dissolved. This is a measure of the molar solubility of the substance in standard conditions and can be found in the internet or in general chemistry textbooks.
  • Step 5: The given equation describes the relationship between Ksp and the solubility of AgCl: Ksp = [Ag][Cl].
  • Step 6: With the equation given above, one should be able to determine the concentrations of the Ag and Cl ions in the saturated solution, by taking the square root of the Ksp value. Square root of Ksp for this example is 1.33 x 10^-5 M, mol per liter. And that is the molar solubility of AgCl.
  • Step 7: The value of each ion concentration is equivalent to the amount of product that has been dissolved. Therefore, the ion concentration is equal to M.


What are the Factors which influence Molar solubility?

The value of M depends on a number of factors.

  • Principally, when the solvent is heated, more energy is available within the system, which allows for greater dissociation of a compound, so molar solubility increases with temperature.
  • If the temperature of a saturated solution is decreased, the M value decreases, and the solute will start to precipitate out of the solution as the temperature decreases.
  • When temperature is decreasing, the concentration of the solution before it begins to precipitate is larger than M, and such a solution is said to be supersaturated. 


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