If 6.50 g of the unknown compound contained 0.217 mol of C and 0.433 mol of H, how many moles of oxygen, O, were in the sample?

First, you should find the mass of carbon and hydrogen in the 6.5g of the unknown compound.

mass of carbon and hydrogen in the compound

= (0.217*12) + (0.433*1)

=3.037g

mass of oxygen in the compound

=6.5-3.037

=3.463g

no of mol of oxygen

=3.463:16

=0.216 mol