Video Transcript
Which of the following occurs when treating a sample of sodium thiosulfate with dilute hydrochloric acid? (A) Formation of a yellow precipitate and evolution of H2S gas. (B) Formation of a yellow precipitate and evolution of SO3 gas. (C) Formation of a yellow precipitate and evolution of SO2 gas. (D) Formation of a black precipitate and evolution of SO2 gas. (E) Formation of a black precipitate and evolution of H2S gas.
The ions present in an unknown chemical substance or mixture can be identified using a process called qualitative analysis. In this process, chemical tests are conducted on a sample and chemical reactions may occur. Observations, such as the evolution of a gas, the production of an odor, the formation of a precipitate, or a color change, can help to identify the ions present in the unknown chemical substance.
This question is asking us to predict the results of a chemical test, in which dilute hydrochloric acid is used to test for the presence of the thiosulfate anion, which has the formula S2O3 2-. When dilute hydrochloric acid is added to a sample of sodium thiosulfate, a solid yellow precipitate is produced along with an acidic gas which is released from the solution. This gas can turn filter paper wetted with potassium dichromate solution from orange to green.
Thiosulfate anions react with hydrogen ions present in the hydrochloric acid solution to produce solid sulfur, sulfur dioxide gas, and liquid water. We can see from the net ionic equation that the solid yellow precipitate is elemental sulfur and that the chemical formula of the acidic gas produced is SO2.
In conclusion, when treating a sample of sodium thiosulfate with dilute hydrochloric acid, the result is the formation of a yellow precipitate and evolution of SO2 gas, or answer choice (C).
