oxidation no of Cr in CrO5 ??? is it +6

Yes indeed, it is +6.

 

 

@Ayisha, Oops!! Wrong this time. It is +6 and not 10. Never mind. Next time answer correctly.

 

First let me draw the structure of CrO₅.

 

By looking at the above structure, you must have observed that 4 of the O atoms attached, are combined by the peroxide linkage. So, there oxidation states are taken as -1.

The other one is normal. So, its oxidation state is taken as -2.

Now, let the oxidation state of Cr be x.
then, x + 4× (-1) - 2 =0

Or, x = 6