Question

An unknown solution MAY contain the following ions:carbonate, sulfate, phosphate, sulfide, chloride iodide and bromide.

a) Upon adding Ba(NO₃)₂, a precipitate formed. Which of the above ions could be present in this solution? Write (full equation or net ionic) equations showing formation of the precipitate(s).

b) After adding nitric acid (HNO₃) to the above precipitate, it completely dissolved, and bubbles were noted in the process. When saturated ammonium molybdate was added, no yellow precipitate was observed. Which of the ions in (a) was present in the solution?

c) The supernatant from part (a) was treated with Cu(NO₃)₂ and a brown/black precipitate formed. This confirms the presence of which ion? Write the equation for the formation of this precipitate (full or net ionic).

d) The supernatant from part (c) was treated Fe(NO₃)₃, and when toluene was added, no color was observed in the toluene layer. However, when KMnO₄ was added, a brown color appeared in the toluene layer. What ions do these tests confirm are present and absent in the unknown solution?

e) Only one ion from the above list has not been tested for in the unknown solution. Which one? Write the chemical equation that tests for this ion in the unknown solution (full equation or net ionic) and the observation that would confirm its presence.

f) Why was the test in part (e) not done immediately after part (c)?

Answer 1

1. a) Upon adding Ba (NO₃)₂, a precipitate formed, carbonate, sulfate and sulfide, could be present in this solution. Salt of barium chloride, barium iodide, barium bromide and are soluble.

   Full equation and net ionic equations showing formation of the precipitate of BaSO₄

   Ba(NO₃)₂(aq) + Na₂SO₄ (aq) ------> BaSO₄(s) + 2 NaNO₃(aq)

   As before, the above equation can be rewritten showing the soluble species as ions in solution:

   Ba2+(aq) + 2 NO₃-(aq) + 2 Na+(aq) + SO₄ 2-(aq) ------> BaSO₄ (s) + 2 Na+(aq) + 2 NO₃-(aq)

   Next, cross out any species which have not changed on both sides of the reactions; these are spectator ions

   What remains is the balanced, net ionic equation:

   Ba2+(aq) + SO₄ 2-(aq) ------> BaSO₄ (s)

   Net ionic equations showing formation of the precipitate of BaS

   Ba2+(aq) + S 2-(aq) ------> BaS (s)

   Net ionic equations showing formation of the precipitate of BaCO₃

   Ba2+(aq) + CO₃ 2-(aq) ------> BaCO₃ (s)

   b) After adding nitric acid to the above precipitate, it completely dissolved, and bubbles were noted in the process. Carbonate anion is present. When saturated ammonium molybdate was added, no yellow precipitate was observed. SO₄ 2- is absent.

   Carbonate anion (CO₃ 2-) is present, which produces bubbles of CO₂ gas

   The net ionic equation for the carbonate anion is:

   2 H+ + CO₃2- → CO₂ + H₂O

   c) The supernatant from part (a) was treated with Cu(NO₃)₂ and a brown/black precipitate formed. This confirms the presence of sulphide ion.

   Cu(NO₃ )₂ + BaS (s)= CuS (s)+ Ba(NO₃)₂(aq)

   d) The supernatant from part (c) was treated Fe(NO₃)₃ and when toluene was added, no color was observed in the toluene layer. However, when KMnO4 was added, a brown color appeared in the toluene layer, indicates the presence of bromide ions and confirm bromide ion is present and iodide ion is absent in the unknown solution

   e) Only chloride ion from the above list has not been tested for in the unknown solution.

   Next, add 15 drops of 0.1 M

   2 AgNO_3(aq) + BaCl₂ (aq)--------> 2 AgCl(s) +Ba(NO₃)₂ (aq)

   AgNO₃ is added to the remaining solution. Formation of a white precipitate indicates the presence of chloride.

   f) The test in part (e) not done immediately after part (c), due toS2- ion. Ag₂S may be precipitated instead of AgCl.