Karen has a solution that contains some mixture of Ba2+, Ca2+, Mg2+, Na+, K+, an

Question

Karen has a solution that contains some mixture of Ba2+, Ca2+, Mg2+, Na+, K+, and NH4+ ions. When she mixes this solution with Na2SO4, she observes no precipitate. however, when she subsequently adds NaOH to the mixture, a precipitate forms. After adding NaOH, Karen heats her sample. This sample then gives off a basic gas. This gas is not given off if she heats her original sample of unknown cations.

i) Based on these observations, what cations are you certain are in the solution? Explain.

ii) Based on these observations, what cations are you certain are not in the solution? Explain.

iii) Based on these obsevations, what cations might or might not be in solution? Explain.

Explanation / Answer

1) Addition of Na2SO4

this means SO4-2 will be in solution

Solubility of SO4-2 compounds:

BaSO4 should precipitate

CaSO4 should precipitate

Since there is no precipitate, there is no Ba+2 or Ca+2 ions

When adding NaOH, there is OH-

NH4+ ions are soluble when OH- is present

Mg+2 will precipitate when OH- is added

Mg+2 + (OH)- --> Mg(OH)2

We are certain that Mg+ is in solution!

When heating, the NH4+ rich OH- solution will form fumes (basic gas)

therefore we are sure there is NH4+ ions as well!

You can't know with this information, know if there is any cation of the Group IA (Na+ and K+)

i)

Cations present: NH4+, Ba+2, Ca+2, Mg+

Catios NOT in solution: NONE

Catios might/not might : Na+, K+

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