Video Transcript
The image shows a series of tests
carried out on an unknown sodium salt, X. What formula is the unknown sodium
salt likely to have?
In this question, we are told that
an unknown sodium salt is being tested with a series of chemical tests. In the image, we see that the
sodium salt is dissolved in a solution. Based on the tests used and the
results of the tests shown in the provided image, we will need to determine the
identity and chemical formula of the sodium salt X. Moving from X to the left, silver
nitrate is added to the solution containing X. The result is a black mixture.
Let’s suppose that the salt X
contains sulfide ions. After adding silver nitrate, the
sulfide ions present in the unknown solution will react with the silver ions from
silver nitrate. The result of the reaction is the
precipitate silver sulfide, which is black in color. The result of the test with silver
nitrate suggests that X contains sulfide ions.
Let’s continue to record the
chemical reactions occurring in the remaining chemical tests. When hydrochloric acid is added to
a fresh sample of the solution containing X, a smelly gas is produced. When hydrochloric acid is added to
the solution containing sulfide ions, a chemical reaction occurs between the sulfide
ions and the hydrogen ions from the acid. The reaction results in the
formation of hydrogen sulfide gas, or H2S, which has the unpleasant odor of rotten
eggs.
According to the diagram, when
lead(II) acetate is added to the mixture that produced the gas, a different black
mixture forms. If there are still sulfide ions or
hydrogen sulfide in the mixture in the test tube, then a chemical reaction will
occur with the lead(II) acetate. The products of the reaction are
acetic acid and lead(II) sulfide, or PbS. Lead(II) sulfide is a black
precipitate that gives the mixture in the test tube its black color.
The results of the three tests
shown in the image give evidence that the sodium salt sample contains sulfide
ions. Therefore, the identity of X is the
salt sodium sulfide, which has the chemical formula Na2S. What formula is the unknown sodium
salt likely to have? Na2S.
