Sodium chlorate, NaClO3, is used industrially as a weedkiller and contains the chlorate ion, ClO3^-. What is the oxidation number of chlorine in the chlorate ion, ClO3^-?

Answer: B. Step 1: Recall the rules needed Two rules are needed here: - Oxygen almost always has an oxidation number of -2. - The oxidation numbers in an ion must add up to the overall charge on the ion. Step 2: Set up the sum for ClO3^- Let the oxid…

Redox Reactions: Question 3

Syllabus 6.4

Multiple choice Extended 1 mark

Sodium chlorate, $\text{NaClO}_3$ , is used industrially as a weedkiller and contains the chlorate ion, $\text{ClO}_3^-$ .

What is the oxidation number of chlorine in the chlorate ion, $\text{ClO}_3^-$ ?

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Step 1: Recall the rules needed

Two rules are needed here:

Oxygen almost always has an oxidation number of $-2$ .
The oxidation numbers in an ion must add up to the overall charge on the ion.

Step 2: Set up the sum for $\text{ClO}_3^-$

Let the oxidation number of chlorine be $x$ . There are three oxygen atoms, each $-2$ , and the ion has an overall charge of $-1$ :

$x + 3(-2) = -1$

Step 3: Solve for $x$

$x - 6 = -1$ $x = -1 + 6 = +5$

Step 4: Check against the options

The oxidation number of chlorine in the chlorate ion is $+5$ , which matches option B. (Option A comes from only counting two oxygens; option C comes from forgetting the ion’s $-1$ charge; option D comes from wrongly assuming chlorine’s oxidation number must equal the ion’s overall charge.)

Final answer

The oxidation number of chlorine in $\text{ClO}_3^-$ is $+5$ , option B.