Precipitating cations with HCl and H2S using solubility rules

[ACLerok]

I'm taking Chem II over the summer at a junior college and was given this problem on one of my homework assignments:

I am told I have a sol'n containing the cations Ag, Al, As, Ba, Bi, Ca, Cd, Co, Cr, Cu, and Fe.

My objective is to write all the compounds that would be precipitated if hydrochloric acid is added to the above sol'n.

My instincts are telling me to just use the solubility rules. For example, when Ag and hydrochloric acid combine, AgCl is formed but according to the solubility rules AgCl is insoluble so AgCl would be one of the compounds that would be precipitated. Am I going about this all wrong or what?

The 2nd part says that a reagent is now needed tha will precipitate the smalest number of remaining cations and that the best reagent for this is hydrogen sulfide plus hydrochloric acid. It then asks me to find all the compounds that would be precipitated.

Do I just go about this the same way for the first part? I don't understand when they say 'smallest number of cations'. Please can anyone help me with this?

 

[Parth Dave]

Yea you are doing it right. If a compound is precipitated than it did not dissolve after combining with the HCl. All you have to do for the first part is find all the compounds that won't dissolve afterwards.

Same thing goes for the second part. The smallest number of cations simply means that a reagant is necessary so that the maximum number of compounds dissolve. In other words, this reagant will make as many compounds dissolve as possible.

 

[M98Ranger]

Your instincts are correct in using the solubility rules to determine which compounds would be precipitated when hydrochloric acid is added to the sol'n. However, it is important to remember that not all compounds will necessarily precipitate. Some may remain in solution, depending on their solubility in the acidic environment.

For the second part, when it says "smallest number of remaining cations," it means that you should choose a reagent that will precipitate the fewest number of cations possible. In this case, hydrogen sulfide plus hydrochloric acid is the best choice because it will precipitate the most cations at once.

To find all the compounds that would be precipitated, you can follow the same method as in the first part. Use the solubility rules to determine which compounds are insoluble in the acidic environment and therefore will be precipitated. Remember to also consider the products of the reaction between the cations and the reagent (in this case, hydrogen sulfide).

I hope this helps and good luck with your Chem II studies!