Answer :

To determine the oxidation state of chlorine (Cl) in [tex]\( HClO_3 \)[/tex], we can follow a systematic approach by using the known oxidation states of the other atoms in the molecule and applying the rule that the sum of oxidation states for all atoms in a neutral molecule must equal zero.

Here are the steps:

1. Identify the known oxidation states:

- Hydrogen (H) usually has an oxidation state of [tex]\( +1 \)[/tex].
- Oxygen (O) usually has an oxidation state of [tex]\( -2 \)[/tex].

2. Write the expression for the sum of the oxidation states:

For the compound [tex]\( HClO_3 \)[/tex], there is one hydrogen atom, one chlorine atom, and three oxygen atoms. Let's denote the oxidation state of chlorine as [tex]\( x \)[/tex].

3. Set up the equation:

The sum of the oxidation states must equal zero for a neutral molecule.

[tex]\[ \text{Oxidation state of } H + \text{Oxidation state of } Cl + 3 \times \text{Oxidation state of } O = 0 \][/tex]

Substituting the known values:

[tex]\[ (+1) + (x) + 3 \times (-2) = 0 \][/tex]

4. Simplify the equation:

[tex]\[ 1 + x - 6 = 0 \][/tex]

5. Solve for [tex]\( x \)[/tex]:

[tex]\[ 1 + x - 6 = 0 \implies x - 5 = 0 \implies x = 5 \][/tex]

6. Conclusion:

The oxidation state of chlorine (Cl) in [tex]\( HClO_3 \)[/tex] is [tex]\( +5 \)[/tex].

So, the correct answer is:
A. +5