Video Transcript
Filter paper wetted with a lead(II)
acetate solution is used as a part of the primary test for an acidic radical. What is this acidic radical? (A) Sulfate, SO4 2−; (B) nitrite,
NO2 1−; (C) sulfide, S2−; (D) phosphate, PO4 3−; or (E) thiosulfate, S2O3 2−.
In chemistry, when we want to know
if a solution or solid contains certain compounds, we perform what is known as a
qualitative test. Typically, the qualitative tests
are chemical reactions that create highly visible indications for the presence of
acidic radicals. The term acidic radical refers to a
negatively charged ion or anion.
This question deals specifically
with a qualitative test using a lead(II) acetate solution on filter paper. An unknown solid can be tested
using dilute hydrochloric acid. If the solid contains sulfide
anions, they react with the hydrogen ions from the acid to form hydrogen sulfide, a
foul-smelling gas. This gas has no color, and we
cannot safely smell it for detection. Because of this, we must use
another reaction to test for the presence of hydrogen sulfide. We can do this using a filter paper
soaked in lead(II) acetate.
When exposed to the hydrogen
sulfide gas, the lead(II) acetate will react. This reaction results in the
formation of a black precipitate, lead(II) sulfide, on the filter paper. So, if this color change is
witnessed, we can say that hydrogen sulfide gas has been produced. We can also indirectly say that
sulfide anions were present in our original sample. It is worth mentioning that to
confirm the presence of sulfide anions, we would need to perform a secondary
confirmatory test that involves another series of reactions.
So, which acidic radical can be
detected in a primary test using filter paper wetted with lead(II) acetate? The correct answer is answer choice
(C) sulfide, S2−.
