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    Home > Chemicals Industry > Chemical Technology > Solubility product rule in precipitation dissolution

    Solubility product rule in precipitation dissolution

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    7.
    5.
    2 Solubility product rule

    Comparing the size of the reaction quotient Q Θ and the solubility product constant Ksp Θ system can determine the direction of the reaction in the poorly soluble strong electrolyte solution
    .

    For the precipitation and dissolution equilibrium in a solution

    The reaction quotient Q Θ of the precipitation and dissolution reaction at a certain moment is

    When Q Θ >Ksp Θ , the solution is supersaturated, and the reaction direction is that precipitation separates out of the solution;

    When Q Θ = Ksp Θ , the solution is saturated and the reaction is in equilibrium;

    When Q Θ <Ksp Θ , unsaturated solution, the reaction direction is the dissolution of the precipitate
    .

    This is the solubility product rule, which is often used to judge the formation and dissolution of precipitates
    .

    [Example 7-9] In 0.
    50mol·dm
    -3 MgCl 2 solution, add an equal volume of 0.
    10 mol·dm
    -3 ammonia water, this ammonia water also contains 0.
    020 mol·dm
    -3 NH 4 Cl, Mg(OH) 2 Can it be precipitated? If precipitation occurs, how many grams of solid NH 4 CI need to be added to each liter of ammonia water to prevent Mg(OH) 2 from precipitation? It is known that Ksp Θ , Mg(OH) 2 =5.
    6×10
    -12 , Kb Θ , NH 3 , H 2 O=1.
    8×10
    -5
    .

    Solution The concentration of each substance after mixing the two solutions in equal volume is

    In NH 3 ·H 2 O-NH4 + mixed solution

    Since Q Θ >Ksp Θ , Mg(OH) 2 precipitation can be generated
    .

    To make the precipitation of Mg(OH) 2 just not formed or just formed, by [Mg 2+ ][OH - ]=Ksp Θ ,Mg(OH) 2 , we get


    According to

    The [NH 4 + ] obtained is the [NH 4 Cl] in the solution when the Mg(OH) 2 precipitation just does not form in the mixed system .
    While the original ammonia


    The mass of solid NH4CI that should be added to each liter of solution is

    53.
    5g·mol
    -1 ×(0.
    38-0.
    020)mol=19.
    26g

    There are many factors that affect the precipitation and dissolution balance, among which the effect of the same ion effect is very obvious, that is, adding a soluble and strong electrolyte with the same ion as the system to the balance system will move the balance to the direction of precipitation, and the solubility of the insoluble strong electrolyte will decrease.

    .
    In addition, the salt effect also has an impact on the precipitation and dissolution balance, which will slightly increase the solubility of insoluble substances, but this effect is generally small

    .

     

     

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