Chemistry
posted by Hannah .
What is the oxidation number of chromium in K2Cr2O7?
I know that K = 1
O=2 so 7*2=14
I do not know what to do now. Thank you for your help!

That's good, you have a good foundation. Since you're only looking for the oxidation number of chromium at this point, let's look at it like this. The polyatomic ion Cr2O7, or dichromate, has a 2 charge. Hence, the charges of the individual ions have to add up to 2. So, we set up an equation, where x is the oxidation number of chromium. In this case, we'll have 2(x) + 7(2) = 2. See what I did there? I set up an equation so that the individual charges will all add up to 2, so let's solve for x, and when you do that, you'll get +6, which happens to be the answer!