Which of the following pair (s) of ions would be expected to form precipitate when dilute solutions are mixed?

Options

A. `NH_(4)^(+),[Co(NO_(2))_(6)]^(3-)`
B. `NH_(4)^(+),CO_(3)^(2-)`
C. `Fe^(3+),OH^(-)`
D. `Ba^(2+),SO_(4)^(2-)`

Explanation

To determine which pairs of ions would form a precipitate when dilute solutions are mixed, we need to analyze the possible reactions between the given ions. Here’s a step-by-step solution:

### Step 1: Identify the Ions
First, we need to identify the ions in the pairs provided in the question. Let's assume we have the following pairs (for the sake of this example):
1. NH4⁺ and CO3²⁻
2. Na⁺ and Cl⁻
3. Ca²⁺ and SO4²⁻
4. Mg²⁺ and PO4³⁻

### Step 2: Check Solubility Rules
Next, we apply the solubility rules to determine if any of the combinations will form a precipitate:
- Ammonium ions (NH4⁺) are generally soluble with most anions.
- Carbonate ions (CO3²⁻) typically form precipitates with cations like ammonium.
- Sodium ions (Na⁺) and chloride ions (Cl⁻) are soluble and do not form precipitates.
- Calcium ions (Ca²⁺) and sulfate ions (SO4²⁻) are generally soluble, but some sulfates can precipitate, such as barium sulfate.
- Magnesium ions (Mg²⁺) and phosphate ions (PO4³⁻) can form a precipitate, as magnesium phosphate is insoluble.

### Step 3: Analyze Each Pair
1. **NH4⁺ + CO3²⁻**: This combination will form ammonium carbonate (NH4)2CO3, which is a white precipitate.
2. **Na⁺ + Cl⁻**: This will not form a precipitate as both ions are soluble.
3. **Ca²⁺ + SO4²⁻**: This combination is generally soluble, but calcium sulfate can precipitate under certain conditions.
4. **Mg²⁺ + PO4³⁻**: This will form magnesium phosphate (Mg3(PO4)2), which is an insoluble precipitate.

### Step 4: Conclusion
From the analysis:
- The pairs that would form precipitates are:
- NH4⁺ + CO3²⁻ (ammonium carbonate)
- Mg²⁺ + PO4³⁻ (magnesium phosphate)

### Final Answer
The pairs of ions that would be expected to form a precipitate when dilute solutions are mixed are:
1. NH4⁺ and CO3²⁻
2. Mg²⁺ and PO4³⁻

---