A solution of Na 2 CO 3 is added dropwise to a solution that is 6.51e-02 M in Nd

Question

A solution of Na2CO3 is added dropwise to a solution that is 6.51e-02 M in Nd3+ and 2.84e-05 M in Hg22+.

The Ksp of Nd2(CO3)3 is 1.08e-33.
The Ksp of Hg2CO3 is 3.6e-17.

Calculate the concentration of CO32- present in the Nd3+/Hg22+ solution when the first (initial) cation begins to precipitate, and the concentration ofCO32- present in this solution when the second (final) cation begins to precipitate. Writedown which cation precipitates first and which precipitates second (last). Choose the correct statement from below based on your work.

THE CORRECT ANSWER IS C BUT CAN YOU EXPLAIN

Explanation / Answer

Nd2(CO3)3 ---------------> 2 Nd3+ + 3 CO32-

  Ksp of Nd2(CO3)3 = [Nd3+]2 [CO32- ]3

1.08*10-33 = [6.51*10-2]2 [CO32- ]3

[CO32-] = 6.35*10-11 M

Hg2CO3 ---------------> Hg22+ + CO32-

Ksp of Hg2CO3 = [Hg+] [CO32- ]

3.6*10-17 = [2.84*10-5] [CO32- ]

[CO32- ] = 1.27*10-12 M

Here 1.27*10-12 M < 6.35*10-11 M .

So, the molarity of CO32- which begins to precipitate the initial cation is 1.27e-12 M

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