[GreekGirl]

  Ηello ^_^

I'm studying for the exams period end i have trouble solving this exercise!

  Solution contains ions 0,025 Μ of Cu^2+ , 0,025Μ of Bi^3+, and 0,025Μ of Cd^2+. It also contains 0.3M HCl. The solution is made saturated by adding H2S.
Which is the concentration of H+  after the precipitation  of the sulfur compounds?

 Τhanksss !

[Arkcon]

Hrm ... seems like a trick question to me, you are given information, but do you need it?

A solution is saturated with H₂S.  Something happens first, but at the end its saturated.

What do you get?  Does the answer key give the number without explanation, and can you match your result to that number, by looking outside the information in the problem?

[GreekGirl]

Ηello Arkcon! First of all thanks for taking the time to answer me!

Ιt says that the solution is saturated, which means that the maximum possible amount of H2S has been dissolved, but there still no precipitation. No  CuS, Bi2S3 and CdS have been formed yet! So the question does not make sense to me ο_ο

[Arkcon]

No  CuS, Bi2S3 and CdS have been formed yet! So the question does not make sense to me ο_ο

You got, then lost it.  So I can be more explicit in helping you.  Follow the order of operations -- the salts are there first, then its is made saturated with H₂S.  I think that means the precip is completed, and hydrogen sulfide is still added, until saturation.  See if trying to solve this problem, as if there were none of those other cations present (because they are all precipitated) gives you the answer you need.  Be sure to account for the HCl present.

[GreekGirl]

Aaghh i do not get it.

[Arkcon]

OK.  What did you try, and what answer did you get?

[GreekGirl]

 I think that we should use the Ksp of each salt, to find out the h2s needed to make a saturated solution, and then compute the h+ from both hcl and h2s.