a.   Elemental analysis : Since the % add up to 100% we know we have a hydrocarbon. The question tells us the mass spectrum showed the molecular ion at m/z =68, so the MW=68 g/mol. Hence one can calculate the empirical formula = C₅H₈

b. In this problem, the molecular formula equals the empirical formula.

c. Index of hydrogen deficiencies = IHD = 2 (this helps us know that the structures need below must contain either a triple bond, 2 double bonds, 1 double bond and a ring or 2 rings).

d.

e.

f.

Common errors:

General: Drawing structures that did not match the molecular formula.

a . Could not do elemental analysis calculations.