Why is it incorrect to write a chemical formula for petrol?

1 Answer
Dec 19, 2016

Answer:

Because petroleum or gasoline is a mixture of isomers.

Explanation:

The gasoline that you put in your petrol motor is #C_5H_12# to #C_7H_16#, a mixture of the isomeric pentanes and hexanes, and a few heptanes. The gas that you put in your diesel motor has one degree of unsaturation, i.e. #C_6H_12# to #C_10H_20#; with the possibility of geometric isomerism, diesel is a bit of a witches' brew.

Most of the time, when we want to represent gasoline, we would use the formula #C_6H_14#, i.e. hexanes; and of course hexanes could be a mixture of isomers: #"dimethylbutanes"#; #"ethylbutane"# #"methylpentanes"#; and #"n-hexane"#.

In the laboratory, often we use so called #"petroleum ethers"#, with various boiling point ranges, #40-60# #""^@C#, #60-80# #""^@C#, #100-120# #""^@C#. These tend to contain some unsaturated material.