What is the oxidation number of Cr in K_2Cr_2O_7 ?

The correct answer is option 3. +6.

Let the oxidation number of \(Cr\) in \(K_2Cr_2O_7\) be \(x\)

We know that,

Oxidation number of \(K\) \(= +1\)

So,

\(2(\text{oxidation number of K}) + 2(\text{oxidation number of Cr}) + 7(\text{oxidation number of O}) = 0\)

 or, \(2(+1) + 2(x) + 7(-2) = 0\)

or, \(+2 + 2x - 14 = 0\)

or, \(2x - 12 = 0\)

or, \(x = \frac{12}{2}\)

or, \(x = 6\)