If 3.50 g of the unknown compound contained 0.117 mol of c and 0.233 mol of h, how many moles of oxygen, o, were in the sample?

To determine the amount of oxygen that is present in the compound, we have to assume that the given compound contains carbon, hydrogen and oxygen only or else we will not be able to determine the answer. We need to convert the moles of the elements given to units of grams by using the atomic mass of these elements. Then, from the total amount of the compound we subtract the masses of the elements. We do as follows:

                                                             mass
0.117 mol C ( 12.01 g / 1 mol ) =        1.41 g  
0.233 mol H ( 1.01 g / 1 mol )   =        0.24 g

Mass O = 3.50 g - 1.41 g - 0.24 g = 1.85 g O
Moles O = 1.85 g O ( 1 mol / 16 g ) = 0.116 moles O

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