Purpose of the Experiment
Determine the presence or absence of each cation in Qualitative Analysis Group 3.
Background Required
You should be familiar with techniques found in the Qualitative Inorganic Analysis Pre- view. You should have the observations from the Group 3 Known for comparisons.
Always Wear Safety Goggles and Use Good Lab Practices
Group 3 Unknown Solution
Your lab instructor will issue you an Unknown. Immediately record the number of your Unknown. Then repeat the Group 3 procedure, substituting your unknown solution for the known cation mixture in step 3–A1. Carefully record all observations.
Not all of the unknown observations will exactly match the observations of the Known solution. For example, the initial color of the known solution was dark indigo. This was a combination of brown (Fe3+), green (Ni2+), very pale pink (Mn2+), and blue-violet (Cr3+) colors. (The Zn2+ cation is colorless, so it does not contribute any color to the known solution.) But the unknown solution will have some cations (and their characteristic colors) absent, so its color will be different from the Known solution. However, the colors and physical states of the confirmation steps should match the observations of the Known. When in doubt if you should omit a procedural step, check with your instructor.
Determinations
Construct a flowchart for your unknown, writing in your observations under the formulas. Remember, some parts of the flowchart will be omitted if you had to omit some procedural steps.
Write a rationale explaining how you determined which cations are absent and which are present. Refer to the Qualitative Inorganic Analysis Preview section online to see an example of a Rationale.
1. For each of the descriptions of different solutions, what inferences can be made? (Which ion(s) are indicated as possibly present, confirmed present, absent, or no conclusion?)
a. KSCN was added to a solution (Fe3+, Ni2+, and/or Mn2+). After stirring, the solution turned to a golden brown solution.
b. NaOH was added to a gold solution (Fe3+, Ni2+, Mn2+, Cr3+, and/or Zn2+). After stirring, a dark brown precipitate formed.
c. When H2O2 was added to a solution ([Zn(OH)4]2– and/or [Cr(OH)4]–), the solution bubbled and turned yellow.
d. NaOH was added to a blue-green solution (Fe3+, Ni2+, Mn2+, Cr3+, and/or Zn2+). After stirring, the solution appeared light green with no precipitate.
2. A student performing the Qual Group 3 Unknown Analysis did not have any precipitate remaining after step 3–E. What steps if any, should the student omit from the procedure? Explain your answer.
3. A student performing the Qual Group 3 Unknown Analysis did not have any precipitate remaining after step 3–A. What steps if any, should the student omit from the procedure? Explain your answer.
4. For each of unknown solutions below, determine what cations are present and which are absent.
a. After adding NaOH in step 3–A to the brown unknown solution, a dark brown precipitate formed with a colorless decantate. After step 3–B, the solution remained colorless. After step 3–C, a light tan solution formed. In step 3–D a blood-red solution formed. A brown precipitate formed in step 3–E with a very pale blue decantate. After step 3–F, a strawberry red solid formed. After step 3–G, there appeared a light gold solution over a mustard brown precipitate. There was no apparent effect on the colorless solution after steps 3–H and 3–I. Finally a white solid appeared in step 3–J.
b. After adding NaOH in step 3–A to the dark blue unknown solution, a pink precipitate formed with a blue decantate. After step 3–B, the solution turned yellow. After step 3–C, a light pink solution formed. In step 3–D, there was no apparent change in the solution. A tan precipitate formed in step 3–E with a colorless decantate. After step 3–F, there was no change in the solution. After step 3–G, a dark purple solution over a mustard brown solid appeared. In step 3–H, a light yellow precipitate formed with a colorless decantate. There was no apparent change in the solution after steps 3–I and 3–J.