Video Transcript
Which of the solutions of sodium salts shown will not produce a gas when reacted with dilute aqueous HCl? (A) Na2S, (B) Na2S2O3, (C) Na3PO4, (D) NaHCO3.
In this question, we want to know which of the sodium salts pictured here will not produce a gas when dilute hydrochloric acid is added to the test tube. In qualitative analysis, we can test for the presence of unknown anions using specific chemical substances in a specific order. First, we use dilute hydrochloric acid. Anions that produce a positive test with hydrochloric acid and produce a gas during the chemical reaction include carbonate, bicarbonate, thiosulfate, sulfide, nitrite, and sulfite ions. If the anion was not identified with hydrochloric acid, then concentrated sulfuric acid is used next. This reagent can detect halides as well as the nitrate ion. Finally, if the anion has still not been identified, then barium chloride solution is used. Sulfate and phosphate anions will react with barium chloride, producing positive tests.
By looking very closely at the chemical formulas of the sodium salts, we can see that test tubes (A), (B), and (D) contain anions that will react with hydrochloric acid. In test tube (A), the sulfide ions will react with hydrogen ions from the hydrochloric acid to produce hydrogen sulfide gas. In test tube (B), the reaction between thiosulfate ions and hydrogen ions produces sulfur dioxide gas along with solid sulfur and water. And in test tube (D), the reaction between bicarbonate ions and hydrogen ions produces carbon dioxide gas along with water.
So, while these three ions will react with hydrochloric acid to produce a gas, the phosphate ions in test tube (C) will not. Phosphate ions are identified with the third testing reagent, barium chloride solution. In conclusion, the sodium salt solution that will not produce a gas when reacted with dilute aqueous HCl is Na3PO4, or the solution in test tube (C).
