The Aromatic Problem - Answers
A little problem that the chemists of the 19th century where presented with.
We are travelling back in time to the early 19th Century, a very exciting time in the world of chemistry. We have carried out a combustion analysis and physical measurements on a clear colourless hydrocarbon isolated from the distillation products of crude oil.
Combustion analysis
Carbon 92.3%
Hydrogen 7.7%
Physical measurements analysis
0.250g of this liquid was vapourised at 100˚C and a pressure of 101.2 kPa. The vapour had a volume of 98cm3. The boiling point was 80˚C and the melting point 5˚C.
Problems
1. What can you deduce from the boiling and melting points about the bonding?
That the structure is simple molecular covalent- low boiling and melting point and there are weak forces between the particles
2. Use the appropriate equation to calculate the Relative molecular mass of this mystery compound.
|
pV = nRT |
V=98cm3 = 0.098dm3 p=101.2 kPa R=8.31 T=373K
|
|
pV/RT = n 101.2 * 0.098/8.31*373 = 3.199x10-3 moles
|
Convert to RMM 0.25g/3.199x10-3 = 78 g mol-1
|
3. Calculate the empirical formula and hence the molecular formula.
|
Element |
|
C |
H |
|
Data |
% |
92.3 |
7.7 |
|
|
RAM |
12.0 |
1.0 |
|
Moles |
=%/RAM |
7.69 |
7.7 |
|
Ratio |
Ratio of moles = divide by smallest |
7.69/7.69 |
7.7/7.69 |
|
|
|
1 |
1 |
|
Empirical formula |
|
C1H1 |
|
|
From RMM = 78 Molecular formula= |
|
C6H6 |
|
Suggest a structure for this compound.
