Question

Given an unknown sample that could contain Cr3 , Mn2 , and/or Fe2 , you strip the metal ions from the chalk using hydrochloric acid, subsequently concentrating the solution and adding sodium hydroxide. You then add sodium hypochlorite and ammonium hydroxide. After boiling and centrifuging you isolate the solution by decantation. You wash the remaining solid twice with water and sodium hydroxide. Water and sulfuric acid are then added to this solid. The decanted solution is yellow, and you add barium chloride, boil and centrifuge. The resulting solid is washed, centrifuged and nitric acid is added. Water is added, and hydrogen peroxide is added to the resulting orange solution, resulting in a blue solution. The solution generated by adding sulfuric acid to the earlier solid is stirred, has hydrogen peroxide added, and boiled. After adding water to the boiled solution, you separate it into two test tubes. To one tube you add potassium thiocyanate, and a deep red color forms. To the second tube, you add a few grains of sodium bismuthate. You mix this solution and centrifuge. You only observe a clear, colorless solution.

Part A
Which cations are present in your unknown sample?
List the metal symbols in alphabetical order, separated by commas. If none, type "none".
Part B
Which group III cations are not present in your unknown sample?
List the metal symbols in alphabetical order, separated by commas. If none, type "none
Part C
Which cations are undetermined in your unknown sample?
List the metal symbols in alphabetical order, separated by commas. If none, type "none".

Answer 1

The analysis indicates the presence of Cr3+ and Fe2+ in the unknown sample. Mn2+ is not present, found by the lack of reaction with sodium bismuthate. Therefore, Cr3+ and Fe2+ are present, Mn2+ is not, and no cations remain undetermined.

Step-by-step explanation:

Based on the described qualitative analysis process, we can infer the presence of certain metal ions in the unknown sample:

• The addition of sodium hydroxide and subsequent treatments that led to a yellow solution and the formation of a blue solution upon the addition of hydrogen peroxide might indicate the presence of Fe2+ ions transitioning to Fe3+ ions (as Fe3+ to Fe2+ can be reversed by hydrogen peroxide).
• The treatment of the solid with sulfuric acid and hydrogen peroxide, followed by the addition of potassium thiocyanate resulting in a deep red color, also suggests the presence of Fe3+, as Fe3+ forms a blood-red complex with thiocyanate ions.
• The lack of a reaction with sodium bismuthate suggests that Mn2+ is not present because Mn2+ would be oxidized to MnO2 (brown solid) by bismuthate.

Consequently, we can conclude:

Part A: The cations present in your unknown sample are Cr3+, Fe2+.

Part B: The group III cation Mn2+ is not present in your unknown sample.

Part C: There are no undetermined cations in your unknown sample.