1. Determine the mass of oxygen:

* Start with the total mass of the compound: 3.50 g

* Subtract the mass of carbon: 3.50 g - 0.117 g = 3.383 g

* Subtract the mass of hydrogen: 3.383 g - 0.233 g = 3.150 g

* This means the mass of oxygen in the sample is 3.150 g

2. Convert mass of oxygen to moles:

* Use the molar mass of oxygen (16.00 g/mol)

* Moles of oxygen = (mass of oxygen) / (molar mass of oxygen)

* Moles of oxygen = 3.150 g / 16.00 g/mol = 0.197 mol

Therefore, there were approximately 0.197 moles of oxygen in the sample.