What mass of oxygen (O2) forms in a reaction that forms 15.90 g C6H12O6?

To answer this question, you need to know the chemical equation for making glucose. It should be look like this

C6H12O6 (s) + 6O2 (g) - - >6CO2 (g) + 6H2O (g)

In the equation, the amount of glucose (C6H12O6) is 1 and the amount of oxygen (O2) is 6. But the equation ratio is using moles as a unit. A mole of glucose will weight 180g but a mole of oxygen weight 32g.

Then, the amount of oxygen forms would be: 6/1 * (15.90g/180g) * 32g = 16.96 grams