Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: impcesar on June 15, 2014, 01:10:26 PM

Title: Simple chemistry question
Post by: impcesar on June 15, 2014, 01:10:26 PM

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+]

Title: Re: Simple chemistry question
Post by: Borek on June 15, 2014, 01:55:25 PM

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

Title: Re: Simple chemistry question
Post by: impcesar on June 15, 2014, 04:40:38 PM

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.

Title: Re: Simple chemistry question
Post by: Borek on June 15, 2014, 04:58:58 PM

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

Title: Re: Simple chemistry question
Post by: impcesar on June 16, 2014, 03:57:56 AM

Quote from: Borek on June 15, 2014, 04:58:58 PM

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?

Title: Re: Simple chemistry question
Post by: Borek on June 16, 2014, 04:51:08 AM

Quote from: impcesar on June 16, 2014, 03:57:56 AM

Isn't there any way to calculate it?

What is unclear about

Quote from: impcesar on June 16, 2014, 03:57:56 AM

No Ka, no solution.

?

Title: Re: Simple chemistry question
Post by: impcesar on June 16, 2014, 04:25:24 PM

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!