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when ammonia is added to Zn(NO3)2 solution, a white precipitate forms, which dissolves on the addition of excess ammonia. But when ammonia is added to a mixture of Zn(NO3)2 and NH4NO3, no precipitate forms at any time. Suggest an explanation for this difference in behaviour.

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3 answers

  1. 1. NH3 + H2O ==> NH4^+ + OH^-
    2. Zn + 2OH^- ==> Zn(OH)2
    3. Zn(OH)2 + NH3 ==> Zn(NH3)4^+2 + 2OH^-
    4. Ksp = (Zn^+)(OH^-)^2
    5. NH4Cl ==> NH4^+ + Cl^-

    NH3 produces OH^- (eqn 1) which combines with Zn (eqn 2) to form Zn(OH)2, a white ppt. An excess of NH3 gives eqn 3 in which the white ppt of Zn(OH)2 dissolves because of the formation of the Znb(NH3)4^+2 ion.

    The fun starts by adding NH4Cl to the NH3 solution. Everything after that goes because of Le Chatelier's Principle. Here we go. Addition of NH4Cl increases the NH4^+ so much it forces the equilibrium of equation 1 to the left. That decreases the OH^- so much that the Ksp for Zn(OH)2 can not be exceeded which prevents the pptn of Zn(OH)2 so no white ppt forms first when NH3 is added.

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  2. thanks!

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  3. This is due to the buffer solution occurred

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