[tashkent]

Greetings, everyone!

I have a problem regarding Ksp.  Assuming that NiS settles in a container and HNO3 is added dropwise, I would like to know how much [H+] concentration is needed before NiS starts to dissolve?  I am given the Ka1 and Ka2 of H2S, being 9.6 x 10-8 and 1.3 x 10-14, respectively.  This was my approach:

1)   Get the [S-2] concentration from NiS, using the Ksp of NiS.  

2)   From the [S-2] you got above, use it to get the [H+] concentration, applying the equation: [H+]2 [S-2] = Ka1x Ka2

This is my problem: there are two Ksp’s for NiS: for ? = 4 x 10-20, and for ? = 1.3 x 10-25.  What Ksp value should I use?

I would appreciate replies from any of you.  Thanks.


Sincerely,
Tashkent

[AWK]

First of all you should decide what does mean "starts to dissolve". Even in water NiS dissolves, but its molar solubility is 2x10-10 M for fresly precipitated NiS, and 3.6x10-13 M for not fresh precipitate. The next problem is that HNO3 dissolve NiS because it remove S2- anions by oxidation, not by  reversion of its dissociation, and only concentrated HNO3 works well.
And finally, you should assume that Ni(HS)2 formed in the course of reaction (its formation is not reliable in this case) is well soluble in water.

[Donaldson Tan]

Alpha and Beta are 2 different crystal lattice structure NiS can take. Each structure is favoured at a different range of P & T.

[AWK]

Alpha and Beta are 2 different crystal lattice structure NiS can take. Each structure is favoured at a different range of P & T.

But sometimes we can meet problem of kinetically or thermodymamically controlled crystalization