How many oxygen atoms are present in 2.5 mol

What does a numerical subscript following an element in a chemical formula mean?

What the blanks in these lines are ?

Design a synthesis of 1-cyano-2-methylbicyclo[2.2.1]-2,5- heptadiene from cyclopentene and any other compounds using a diels-alder cycloaddition reaction. part 1 out of 9 choose the best option for the dienophile precursor to the target molecule.