Select the incorrect statement(s):

Options

A. KOH is a weaker than NaOH
B. Milk of magnesia is an aqueous solution of `Mg(OH)_(2)`
C. `Mg^(2+)` ions are precipitated with the addition of `NH_(3)OH` in the presence of `NH_(4)Cl`
D. `CaO_(2)` is less stable than `MgO_(2)`

Answer

To solve the question of identifying the incorrect statements regarding s-block elements, we will analyze each statement step by step.

### Step 1: Analyze the first statement
**Statement:** KOH is weaker than NaOH.

**Analysis:**
- KOH (potassium hydroxide) and NaOH (sodium hydroxide) are both strong bases.
- As we move down the group in the alkali metals, the basicity of hydroxides increases due to the decrease in ionization energy.
- Therefore, KOH is actually a stronger base than NaOH.

**Conclusion:** This statement is incorrect.

### Step 2: Analyze the second statement
**Statement:** Milk of magnesia is an aqueous solution of Mg(OH)₂.

**Analysis:**
- Milk of magnesia is indeed a suspension of magnesium hydroxide (Mg(OH)₂) in water.
- It is commonly used as an antacid and is correctly described as an aqueous solution of Mg(OH)₂.

**Conclusion:** This statement is correct.

### Step 3: Analyze the third statement
**Statement:** Mg²⁺ ions are precipitated with the addition of NH₃OH in the presence of NH₄Cl.

**Analysis:**
- NH₃OH refers to ammonium hydroxide, which can release OH⁻ ions in solution.
- However, the presence of NH₄Cl introduces NH₄⁺ ions, which suppress the ionization of NH₃, leading to a lower concentration of OH⁻ ions.
- This means that the concentration of OH⁻ ions may not be sufficient to precipitate Mg²⁺ ions effectively.

**Conclusion:** This statement is incorrect.

### Step 4: Analyze the fourth statement
**Statement:** CaO₂ is less stable than MgO₂.

**Analysis:**
- The stability of peroxides generally decreases as we move down the group in the periodic table.
- Therefore, CaO₂ (calcium peroxide) is indeed less stable than MgO₂ (magnesium peroxide).

**Conclusion:** This statement is correct.

### Final Conclusion
The incorrect statements are:
- A: KOH is weaker than NaOH (Incorrect)
- C: Mg²⁺ ions are precipitated with the addition of NH₃OH in the presence of NH₄Cl (Incorrect)
- D: CaO₂ is less stable than MgO₂ (Incorrect)

Thus, the correct answer is A, C, and D.