Question Video: Identifying the Counterion of Sodium in a Salt Based on Chemical Tests Chemistry

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?

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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.

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