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 =70, so the MW=70 g/mol. Hence one can calculate the molecular formula = C_{5}H_{10}

**b.**
**Index
of hydrogen deficiencies** = IHD = 1

**c.**** A **and **B **are acyclic (i.e no rings) so the IHD=1 implies we are looking at alkenes. The spectral data tells us that **A** has 5 H types and 4 C types and** B** has 4 H types and 5 C types (hint **B** has a C without any H attached). Note that in **A** the two H in the H_{2}C=C are not equivalent as one is *cis* to the isopropyl group and the other is* trans*.

**d.**

**e.** We can determine which of **A** and **B** are more stable by comparing their heats of formation or combustion. Since the heat of formation of **B** is more exothermic than A, then **B **must be more stable than **A**.

**Common errors: **

**General: **Breaking valence rules (too many bonds, incorrect charges etc.).
Incorrect structural representations such as when using line diagrams or being
inconsistent when showing H atoms (i.e. showing some but not all)

** a **. Could not do elemental analysis calculations.

**c**. Did not know what acyclic meant and so drew cyclic structures.