Part 6: STRUCTURE DETERMINATION

EA:
The data adds up to 100% so there are no "hidden elements" but we don't know the molecular weight. That doesn't matter because the data can still provide the empirical formula (simplest ratio) by considering using a 100g and remembering NEVER TO ROUND DATA during EA calculations (it will invariably mean you get the wrong answer).

%C = 83.24  divide by atomic weight :  83.24/12.011 = 6.9303 moles
%H = 16.76  divide by atomic weight :  16.76/1.008 = 16.627 moles

So we have 6.9303 moles of C for every 16.627 moles of H, by dividing by the smallest, that tells us for each C there are 2.4 H atoms.... or C1H2.4  but that is not an empirical formula, the simplest integer ratio is C5H12. The easiest way to get this is to multiply by 10 first to get rid of the decimal then recognise that you can divide by 2.

So the empirical formula = C5H12. Since there are only 3 isomers, this means this is also the molecular formula.

Why the boiling point order ?
Physical properties are determined by intermolecular forces (i.e. the forces between molecules) and are NOT connected to the thermodynamic stability (which is primarily governed by the intramolecular forces such as the covalent bonds).
There are 3 types of intermolecular forces : London dispersion forces, dipole-dipole forces and hydrogen bonding.
In the alkanes for this question we are only talking about the London dispersion forces since we lack polar bonds (so no dipole-dipole interactions) and no H atoms on electronegative N or O atoms (so no H-bonding). Non-branched structures are essentially linear in shape. A more branched structure is more spherical in shape and therefore has less surface contacts with neighbouring molecules. This means there are less intermolecular forces and so less energy (hence lower temperature) is required to separate the molecules during the boiling process.