I need help figuring out how to set this problem up to find the simplest formula for the compound. I know that the correct answer should be CrO2, but I've setup the problem every way I know how to and I still can't get the Oxygen to 2. I need help setting up the problem to get the correct answer. Please help! : )

First you need to calculate the mass of oxygen in the sample.

Secondly calculate the number of moles of each element by dividing by their respective atomic weights.
(A formula shows the ratio of number of atoms of each element which equals the ratio of number of moles of each element)

Lastly simplify the ratio of number of moles to get the answer.

I need to know how to set the problem up, I already know what I need to do, I just can't get the correct formation of the problem to get the correct answer. Thank you for your post though!

Post what you did till now. I'm a little confused of what you're saying you don't know what to do.

You can put your work in a simple table with headings O and Cr, putting their masses below each heading, then below each mass the respective number of moles, and again below that, you put the mole ratio. You should get 1.99838 and 1 for the mole ratio, which simplifies to 2 and 1, giving CrO2

I had calculated Cr just fine, but for some reason I was getting 5 for O... I finally figured out that I need to subtract 3.09 from 4.99 to get the difference ratio for O and then to divide by the smalled denominator which was Cr. I appreciate all of you help though! Thank you! : )

Yes, that's why elscarta said first of all, calculate the mass of oxygen :)