The pair of compounds which cannot exist together in aqueous solution is:
`(I) NaH_(2)PO_(4)` and `NaHCO_(3)" "(II) Na_(2)CO_(3)` and `NaHCO_(3)`
`(III) NaOH` and `NaH_(2)PO_(2)" "(IV) NaHCO_(3)` and `NaOH`

Options

A. I, II, III
B. II, III
C. III, IV
D. only IV

Answer

To determine which pair of compounds cannot exist together in aqueous solution, we need to analyze the nature of each compound and their possible interactions. Let's go through each pair step by step.

### Step 1: Analyze the first pair `(I) NaH2PO4 and NaHCO3`
- **NaH2PO4** (sodium dihydrogen phosphate) is an acidic salt.
- **NaHCO3** (sodium bicarbonate) is also an acidic salt.
- **Conclusion**: Acidic salts do not react with each other, so they can coexist in solution.

### Step 2: Analyze the second pair `(II) Na2CO3 and NaHCO3`
- **Na2CO3** (sodium carbonate) is a basic salt.
- **NaHCO3** (sodium bicarbonate) can act as a weak acid.
- **Conclusion**: A basic salt and a weak acid can coexist without reacting significantly, so they can exist together in solution.

### Step 3: Analyze the third pair `(III) NaOH and NaH2PO2`
- **NaOH** (sodium hydroxide) is a strong base.
- **NaH2PO2** (sodium dihydrogen phosphite) can act as a weak acid.
- **Conclusion**: A strong base and a weak acid can coexist without reacting significantly, so they can exist together in solution.

### Step 4: Analyze the fourth pair `(IV) NaHCO3 and NaOH`
- **NaHCO3** (sodium bicarbonate) is a weak acid.
- **NaOH** (sodium hydroxide) is a strong base.
- **Reaction**: When they are mixed, they react to form sodium carbonate (Na2CO3) and water.
- **Conclusion**: Since they react with each other, they cannot coexist in solution.

### Final Answer
The pair of compounds which cannot exist together in aqueous solution is **(IV) NaHCO3 and NaOH**.

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