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 = C_{5}H_{8}

**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.