Which of the following oxidation numbers is not correctly matched ?

Options

A. Cl in `HClO_(4)`
B. Ni in `[Ni(CN)_(6)]^(4-)=+2`
C. P in `Mg_(2)P_(2)O_(7)=+6`
D. Cr in `(NH_(4))_(2)Cr_(2)O_(7)=+6`

Explanation

To determine which oxidation number is not correctly matched, we will analyze each compound one by one and calculate the oxidation states accordingly.

### Step-by-Step Solution:

1. **Analyzing HClO4 (Perchloric Acid)**:
- Let the oxidation number of chlorine be \( x \).
- The oxidation number of hydrogen (H) is \( +1 \) and for oxygen (O) it is \( -2 \).
- The equation for the compound is:
\[ +1 + x + (-2 \times 4) = 0 \]
- Simplifying this gives:
\[ +1 + x - 8 = 0 \implies x - 7 = 0 \implies x = +7 \]
- Therefore, the oxidation number of chlorine in HClO4 is \( +7 \).

2. **Analyzing Ni(CN)6^4- (Nickel Complex)**:
- Let the oxidation number of nickel (Ni) be \( x \).
- The oxidation number of the cyanide ion (CN) is \( -1 \).
- The total charge of the complex is \( -4 \).
- The equation is:
\[ x + (-1 \times 6) = -4 \]
- Simplifying gives:
\[ x - 6 = -4 \implies x = +2 \]
- Therefore, the oxidation number of nickel in Ni(CN)6^4- is \( +2 \).

3. **Analyzing Mg2P2O7 (Magnesium Phosphate)**:
- Let the oxidation number of phosphorus (P) be \( x \).
- The oxidation number of magnesium (Mg) is \( +2 \) and for oxygen (O) it is \( -2 \).
- The equation is:
\[ 2(+2) + 2x + (-2 \times 7) = 0 \]
- Simplifying gives:
\[ 4 + 2x - 14 = 0 \implies 2x - 10 = 0 \implies 2x = 10 \implies x = +5 \]
- Therefore, the oxidation number of phosphorus in Mg2P2O7 is \( +5 \).

4. **Analyzing (NH4)2Cr2O7 (Ammonium Dichromate)**:
- Let the oxidation number of chromium (Cr) be \( x \).
- The oxidation number of ammonium (NH4) is \( +1 \) and for oxygen (O) it is \( -2 \).
- The equation is:
\[ 2(+1) + 2x + (-2 \times 7) = 0 \]
- Simplifying gives:
\[ 2 + 2x - 14 = 0 \implies 2x - 12 = 0 \implies 2x = 12 \implies x = +6 \]
- Therefore, the oxidation number of chromium in (NH4)2Cr2O7 is \( +6 \).

### Conclusion:
- The calculated oxidation states are:
- HClO4: Cl = +7
- Ni(CN)6^4-: Ni = +2
- Mg2P2O7: P = +5
- (NH4)2Cr2O7: Cr = +6

After comparing these calculated values with the provided options, we find that all the oxidation states are correctly matched except for the phosphorus in Mg2P2O7, which is stated incorrectly in the question.

### Final Answer:
The oxidation number that is not correctly matched is for phosphorus in Mg2P2O7.

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