You are given 12.35g of c4h8o2 how many moles of compound do you have?

The molecular mass of C4H8O2 is: 

  (4 atoms of carbon) • (12 g/atomic mole) = 48 g/molecular mole 
   (8 atoms of carbon) • (1 g/atomic mole) = 8 g/molecular mole 
  (2 atoms of carbon) • (16 g/atomic mole) = 32 g/molecular mole 
                 TOTAL = 88 g/mole_C4H8O2 

   (12.35 g_C4H8O2)  ⁄  (88 g/mole_C4H8O2) = 0.14 moles_C4H8O2 

For a more precise answer use the three exact atomic masses: 
         example: 12.0107 g/atomic mole_carbon 
       15.9994 g/atomic mole_oxygen 
        1.00794 g/atomic mole_hydrogen