Molar solubility is the maximum number of moles of a solute that can dissolve in one liter of solvent. It depends on the solubility product constant and stoichiometry of the reaction. To calculate M, balance the chemical equation, find the Ksp value, and use the equation Ksp=(Na)(Cl) to find the ion concentrations, which are equivalent to M. M increases with temperature and decreases as the solution becomes supersaturated.
Molar solubility (M) is a measure of the ability of a compound, called a solute, to dissolve in a specific substance, called a solvent. Specifically, it is the maximum number of moles of a solute that are capable of dissolving in one liter of solvent, so molar solubility is measured as moles/L. When the number of moles dissolved in a liter of solvent equals the molar solubility, the solution is said to be saturated, meaning it cannot dissolve any more solute.
The value of M depends on a number of variables. Of importance is the solubility product constant, which is designated Ksp. This value is constant for any compound which describes how easily a substance dissolves. Also, the ratio of products to reactants in the chemical reaction must be known to calculate the value of M. This is referred to as the stoichiometry of the reaction.
The first step in calculating molar solubility is to balance the chemical equation for the dissociation of the substance. To do this, you need to make sure that there are equal numbers of each atom on both the product side and the reactant side of the chemical reaction. An example of a balanced chemical equation with equal numbers of each atom on the product and reactant sides is shown by the dissociation of a molecule of sodium chloride (NaCl) into atoms of sodium (Na) and chlorine (Cl): NaCl (s) = Na+ (aqueous) + Cl- (aqueous). The physical state of atoms and molecules is designated using s for solid and aq for aqueous or dissolved in water.
To determine M, you will need to look up the Ksp value for the starting solute or compound that is dissolving. This is a measure of the molar solubility of the substance under standard conditions and can be found online or in chemistry textbooks. The following equation describes the relationship between Ksp and the solubility of NaCl: Ksp = (Na)(Cl)
Using this equation, you will be able to find the concentrations of Na and Cl ions in the saturated solution by taking the square root of the Ksp value. 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.
The value of M depends on a number of factors. Specifically, 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 decreases, the M-value decreases and the solute will begin to precipitate out of the solution as the temperature decreases. As the temperature decreases, the concentration of the solution before it begins to precipitate is greater than M, and such a solution is said to be supersaturated.
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