**Cr in Cr2O7 -2 what is the oxidation number**

**Answer:**

Start by writing your charge balance equation (there are two unknowns so “X” is the oxidation number of Cr and Y of Oxygen.

-2[Cr2O7] = X[Cr] + Y[O]

fill in the quantities for each species

-2[1] = X[2] + Y[7]

Oxygen has an oxidation number of -2 in all but a few rate cases so it is safe to assume it into the equation.

-2[1] = X[2] + -2[7]

-2 = X[2] + (-14)

X[2] = -2 - (-14)

X = 12/2

X=6

+6 is the oxidation number of Cr.