In which of the following compounds iron has lowest oxidation number?

Options

A. `FeSO_(4).(NH_(4))_(2)SO_(4).6H_(2)O`
B. `K_(4)[Fe(CN)_(6)]`
C. `Fe_(2)O`
D. `Fe_(2)O_(3)`

Answer

To determine the compound in which iron has the lowest oxidation number, we will analyze the oxidation states of iron in each of the given compounds. Let's denote the oxidation number of iron as \( x \).

### Step 1: Analyze each compound

1. **Compound 1: \( \text{(NH}_4)_2\text{SO}_4 \) (Ammonium sulfate)**
- The ammonium ion \( \text{NH}_4^+ \) has a charge of +1.
- There are 2 ammonium ions: \( 2 \times +1 = +2 \).
- The sulfate ion \( \text{SO}_4^{2-} \) has a charge of -2.
- The overall charge of the compound is 0.
- Setting up the equation:
\[ 2(+1) + x + (-2) = 0 \]
\[ 2 + x - 2 = 0 \implies x = 0 \]

2. **Compound 2: \( \text{K}_4\text{Fe(CN)}_6 \) (Potassium ferrocyanide)**
- The potassium ion \( \text{K}^+ \) has a charge of +1.
- There are 4 potassium ions: \( 4 \times +1 = +4 \).
- Each cyanide ion \( \text{CN}^- \) has a charge of -1, and there are 6 cyanide ions: \( 6 \times -1 = -6 \).
- The overall charge of the compound is 0.
- Setting up the equation:
\[ 4 + x + (-6) = 0 \]
\[ 4 + x - 6 = 0 \implies x = +2 \]

3. **Compound 3: \( \text{Fe}_2\text{O}_3 \) (Iron(III) oxide)**
- Oxygen has an oxidation state of -2.
- There are 3 oxygen atoms: \( 3 \times -2 = -6 \).
- The overall charge of the compound is 0.
- Setting up the equation:
\[ 2x + (-6) = 0 \]
\[ 2x - 6 = 0 \implies 2x = 6 \implies x = +3 \]

4. **Compound 4: \( \text{Fe}_2\text{O} \) (Iron(I) oxide)**
- Oxygen has an oxidation state of -2.
- There is 1 oxygen atom: \( 1 \times -2 = -2 \).
- The overall charge of the compound is 0.
- Setting up the equation:
\[ 2x + (-2) = 0 \]
\[ 2x - 2 = 0 \implies 2x = 2 \implies x = +1 \]

### Step 2: Summary of oxidation states
- In \( \text{(NH}_4)_2\text{SO}_4 \), \( x = 0 \)
- In \( \text{K}_4\text{Fe(CN)}_6 \), \( x = +2 \)
- In \( \text{Fe}_2\text{O}_3 \), \( x = +3 \)
- In \( \text{Fe}_2\text{O} \), \( x = +1 \)

### Conclusion
The lowest oxidation number of iron occurs in the compound \( \text{(NH}_4)_2\text{SO}_4 \), where the oxidation number of iron is **0**.

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