For the reaction `CrO_(4)^(2-)+?rarrCr_(2)O_(7)^(-2)`  the missing ion is

Options

A. `OH^(-)`
B. `H^(+)`
C. `OH^(+)`
D. `O^(2-)`

Answer

To solve the question regarding the missing ion in the reaction \( \text{CrO}_4^{2-} + ? \rightarrow \text{Cr}_2\text{O}_7^{2-} \), we can follow these steps:

### Step 1: Identify the Reaction Type
The reaction involves the conversion of chromate ion (\( \text{CrO}_4^{2-} \)) to dichromate ion (\( \text{Cr}_2\text{O}_7^{2-} \)). This is a redox reaction where the oxidation state of chromium changes.

**Hint:** Look for the changes in oxidation states to determine if it's a reduction or oxidation reaction.

### Step 2: Determine the Missing Ion
In this reaction, \( \text{CrO}_4^{2-} \) is being converted to \( \text{Cr}_2\text{O}_7^{2-} \). To balance the charge and the number of oxygen atoms, we need to add protons (\( \text{H}^+ \)) to the reaction.

**Hint:** Consider what ions are typically involved in acid-base reactions and redox reactions.

### Step 3: Write the Balanced Reaction
The balanced reaction can be written as:
\[ 2 \text{CrO}_4^{2-} + 2 \text{H}^+ \rightarrow \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \]

This shows that two moles of chromate react with two moles of hydrogen ions to form one mole of dichromate and water.

**Hint:** Balancing the reaction often involves ensuring that both the number of atoms and the charges are equal on both sides.

### Conclusion
The missing ion in the reaction is \( \text{H}^+ \).

**Final Answer:** The missing ion is \( \text{H}^+ \).

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