A : Oxidation number of carbon in HCN is +4.
R : Oxidation state and valency is same for carbon.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To determine the correctness of the statements provided in the question, we will analyze each statement (A and R) step by step.

**Step 1: Determine the oxidation number of carbon in HCN.**

1. The molecular formula of hydrogen cyanide (HCN) consists of hydrogen (H), carbon (C), and nitrogen (N).
2. We assign oxidation states based on known values:
- Hydrogen (H) typically has an oxidation state of +1.
- Nitrogen (N) in HCN is usually assigned an oxidation state of -3.
3. Let the oxidation state of carbon (C) be represented by \( x \).
4. The sum of the oxidation states in a neutral molecule must equal zero:
\[ (+1) + x + (-3) = 0 \]
5. Simplifying this equation:
\[ x - 2 = 0 \implies x = +2 \]

**Conclusion for Statement A:**
The oxidation number of carbon in HCN is +2, not +4. Therefore, Statement A is **false**.

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**Step 2: Analyze the relationship between oxidation state and valency for carbon.**

1. The oxidation state of an element refers to the degree of oxidation (loss of electrons) of that element in a compound.
2. Valency refers to the ability of an atom to bond with other atoms, which is often determined by the number of electrons an atom can lose, gain, or share.
3. For carbon:
- The common valency of carbon is 4, as it can form four covalent bonds by sharing its four valence electrons.
- The oxidation state can vary depending on the compound it is in. In HCN, the oxidation state of carbon is +2, which is different from its maximum valency of 4.
4. Therefore, the statement that "oxidation state and valency is the same for carbon" is incorrect.

**Conclusion for Statement R:**
The oxidation state and valency are not the same for carbon. Therefore, Statement R is **false**.

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**Final Conclusion:**
Both statements A and R are false.

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