Suppose you have a solution that might contain any or all of the following cations: Ni2+, Ag+, Sr2+, and Mn2+. Addition of HCl solution causes a precipitate to form. After filtering off the precipitate, H2SO4 solution is added to the resulting solution and another precipitate forms. This is filtered off, and a solution of NaOH is added to the resulting solution. No precipitate is observed. Which of the four ions listed above must be absent from the original solution?

Verified step by step guidance

1. Step 1: Identify the cations that form precipitates with HCl. Ag+ forms a precipitate with Cl- as AgCl, which is insoluble in water. Therefore, if a precipitate forms upon adding HCl, Ag+ is present.
2. Step 2: Consider the cations that form precipitates with H2SO4. Sr2+ forms a precipitate with SO4^2- as SrSO4, which is insoluble in water. If a precipitate forms upon adding H2SO4, Sr2+ is present.
3. Step 3: Analyze the cations that form precipitates with NaOH. Ni2+ and Mn2+ can form precipitates with OH- as Ni(OH)2 and Mn(OH)2, respectively, which are insoluble in water. If no precipitate forms upon adding NaOH, both Ni2+ and Mn2+ must be absent.
4. Step 4: Conclude which ions must be absent. Since no precipitate forms with NaOH, both Ni2+ and Mn2+ are absent from the original solution.
5. Step 5: Summarize the findings. The absence of a precipitate with NaOH indicates that Ni2+ and Mn2+ are not present in the original solution.