A mole of anything contains #6.022xx10^23# individual items of that something. You have 3 moles, so there are #3xx6.022xx10^23# #"oxygen molecules"#. How many #"oxygen atoms"# have you got?

Why should we use such an absurdly large number? Well, it turns out that if I have #N_A# #""^1H# atoms, I have a mass of #1*g# precisely. The mole is thus the link between the MICRO world of atoms and molecules, about which we can theorize, and the MACRO world of grams, and kilograms, and litres, and cubic metres, that which we can actually measure on a laboratory bench. If 1 mole of dioxygen gas has a mass of #32*g#, what is the mass of your quantity?