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With the only possibilities being $\ce{Ba^2+, Ca^2+, Cu^2+, Fe^2+, Fe^3+, Pb^2+}$, what cations are present in a solution which:
a) Forms a white precipitate with sulfate and with fluoride, no precipitate with chloride, and a blue precipitate with hydroxide which dissolves in ammonia solution

Salt analysis chart for determining cations present in a solution

I have the answer with me, but I am not too sure how to use the above chart for determining cations in a solution, to answer the question. I can sort of make it out, but I am not completely sure.

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    $\begingroup$ What is your thinking? Please include that detail into your question $\endgroup$
    – user15489
    Sep 6 '15 at 5:56
  • $\begingroup$ @santiago yeah i'm not really sure..I can see the precipitates that form as mentioned in the question but I'm having trouble linking them all together...I think I'm over complicating this in a way or another $\endgroup$ Sep 6 '15 at 5:58
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For this question rather than reading the question and trying to understand what is going on all in one go, I suggest just work your way through the flow chart and refer to the question as you go along so that you don't overload your brain with excess information and confuse yourself.

So starting from the top, it asks if a precipitate will form if $\ce{HCl}$ is added. Looking from the information in the question, there is no precipitate formed. Therefore we can rule out $\ce{Pb^2+}$ being in the solution. Now following the steps in the flow chart, it asks if a precipitate forms if sulfuric acid is added to it. Looking from the data in the question, it says that a white precipitate is formed with sulfate. Therefore, we follow the next step which asks if a precipitate is formed when $\ce{NaF}$ is added. In the question it states that a white precipitate forms with fluoride. Therefore we can conclude that $\ce{Ca^2+}$ ions are in the solution.

However, if we look again into the question, it states that a blue precipitate also forms when mixed with hydroxide ions. Therefore, if we look back to the flow chart regarding the section on testing with $\ce{NaOH}$, it can be seen that traces of $\ce{Cu^2+}$ ions also exist in the solution. Therefore, both $\ce{Ca^2+}$ and $\ce{Cu^2+}$ cations are in the solution.

As you can see, once you know the general method, it is really quite simple and self-explanatory.

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  • $\begingroup$ The blue precipitate from the hydroxide suggests the presence of Cu2+ as well $\endgroup$
    – user15489
    Sep 6 '15 at 6:07
  • $\begingroup$ Yep, you right, I didn't read the question properly. I didn't read 'cations'. $\endgroup$
    – Nanoputian
    Sep 6 '15 at 6:09
  • $\begingroup$ No problems, edit it in to your answer if you'd like. $\endgroup$
    – user15489
    Sep 6 '15 at 6:10
  • $\begingroup$ There could also be some $\ce{Ba^2+}$ mixed in; the chart cannot distinguish $\ce{Ba^2+ + Ca^2+}$ from just $\ce{Ca^2+}$. $\endgroup$ Sep 6 '15 at 14:12

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