General Chemistry 2

Question

Suppose you are studying an unknown solution based on its precipitation reactions with other solutions, resulting in this data table.

Na2SO4	NaCl	Na2CO3.
Unknown	Precipitate	No reaction	Precipitate

Na2SO4 forms a precipitate, NaCl has no reaction and Na2CO3 has a precipitate

Which compounds might be your unknown?Select one or more:

1. 
   
2. NaNO3
3. 
   
4. Ba(NO3)2
5. 
   
6. Sr(NO3)2
7. 
   
8. Pb(NO3)2
9. 
   
10. Ca(NO3)2
11. 
    
12. AgNO3

Answer 1

In order to answer this question, we are going to look at a solubility chart for each reaction. The solubility chart (maybe google one and follow along with this problem) tells you which elements are soluble together and which will form a precipitate (solid that falls out of the solution). When the unknown compound undergoes double replacement with Na2SO4 and Na2CO3, a precipitate forms, but this does not occur when the unknown reacts with NaCl. (When there is no precipitate or gas formed, it is considered no reaction). This means we need to find what elements/compounds are insoluble in SO4 and CO3 but are soluble in Cl.

Compounds that are insoluble with SO4: Ba, Sr, Ca, Pb, Ag, Hg

Compounds that are insoluble with Cl: Ag, Pb, Hg

Compounds that are insoluble with CO3: everything except Na, K, and NH4

We are looking for an unknown that is in the first and third lists, but not the second. This will ensure a precipitate with sulfate and carbonate but no reaction with chlorine.

NaNO3 —> Na is not insoluble according to any of the lists so it will not cause any reaction and is not correct

Ba(NO3)2 —> Ba is on the lists for SO4 and CO3 but not in the list for Cl, therefore barium nitrate follows the results in the table and would be a good option

Pb(NO3)2 —> Pb is in every list, so this would form a precipitate in all the reactions and therefore is not correct

Ca(NO3)2 —> Ca is in the lists for only SO4 and CO3 and therefore is a good option

AgNO3 —> Ag is in all 3 lists so it would make a precipitate in every scenario and is not correct