[impcesar]

Hi,

I am having problems with this exercise:

Data:
S (FeS) = 6,3 x 10^-18 (25ºC)

Precipitation equation:
Fe2+ (aq) + S2–(aq) <> FeS(s)


Question:
We want to precipitate FeS from a solution of Fe2+(aq) with 4,47g/dm-3 using H2S, with the concentration of 0,10mol/dm-3, which is maintained during the reaction.
M(Fe2+) = 55,85g mol-1

-> Calculate the Hidrogen ion concentration necessary so that the FeS can precipitate.


what I already did:

Concentration of Fe: 0,08mol/dm-3
Concentration of Sulfur: 7,88*10^-17 mol dm-3

What I tried to do:

Ka = [S2-].[H3O+]^2/[H2S]

The problem is that neither I don't know nor is given the Ka, and I don't know how to calculate it, so that I can use the above equation to calculate [H3O+]

[Borek]

You have to check it somewhere. Perhaps it is in the back of the book.

[impcesar]

It isn't. That is only what they give me. On the solutions page they have the Ka, but they don't show how to calculate it (they are full resolutions, not only the final answer).

This question was on the national examsof 2012, they would not make a mistake in one of the most important exams.

Edit: and if they did, it would be on the book.

[Borek]

Then there is something wrong with the question wording or with the source. No Ka, no solution.

[impcesar]

Then there is something wrong with the question wording or with the source. No Ka, no solution.

Ok, thank you

Isn't there any way to calculate it? They say that the temperature is 25ºC, therefore Kc is 1,0x10^-14

They give us the concentration of the acid and they say that it is maintained through the reaction (0,10mol dm-3)

Would calculating the Kc help me calculating the Ka?

[Borek]

Isn't there any way to calculate it?

What is unclear about

No Ka, no solution.

?

[impcesar]

Ok, I called the publisher of the book and they said they said they had some printing error or something like that. The missing information was between the two pages and was not printed (what?)

So... thank you.

Also, @Borek, don't need to be so harsh mate!