The stoichiometric air-fuel ratio of petrol is roughly 15:1. This means that for every 15 parts of air, 1 part of fuel is needed for complete combustion.

Explanation:

1. Stoichiometric Air-Fuel Ratio:
The stoichiometric air-fuel ratio is the ideal ratio of air to fuel that allows for complete combustion without any excess oxygen or unburned fuel. It is the point where all the fuel is burned with the available oxygen, resulting in the most efficient and complete combustion process.

2. Petrol Composition:
Petrol, also known as gasoline, is a mixture of hydrocarbons primarily derived from crude oil. It consists of various compounds such as octane, heptane, and other hydrocarbons with different molecular structures. The exact composition of petrol may vary, but it generally contains carbon and hydrogen atoms.

3. Combustion Process:
When petrol is burned in the presence of oxygen, a chemical reaction called combustion occurs. During this process, the carbon and hydrogen atoms in the petrol combine with oxygen to produce carbon dioxide (CO2) and water vapor (H2O). The combustion reaction can be represented by the following equation:

C8H18 + 12.5(O2 + 3.76N2) -> 8CO2 + 9H2O + 47N2

In this equation, C8H18 represents the octane molecule, O2 represents oxygen, N2 represents nitrogen (a major component of air), CO2 represents carbon dioxide, H2O represents water vapor, and 47N2 represents the excess nitrogen that is present in air.

4. Stoichiometric Ratio Calculation:
To determine the stoichiometric air-fuel ratio, we need to balance the equation by ensuring that the number of atoms is the same on both sides. By doing so, we find that for every 1 mole of octane, we need 12.5 moles of oxygen and 47 moles of nitrogen. Since air is composed of approximately 21% oxygen and 79% nitrogen by volume, we can calculate the stoichiometric air-fuel ratio as follows:

(12.5/21) * (1/0.21) = 14.88

Rounding it off, we get approximately 15:1 as the stoichiometric air-fuel ratio.

Conclusion:
In conclusion, the stoichiometric air-fuel ratio of petrol is roughly 15:1. This means that for every 15 parts of air, 1 part of petrol is required for complete combustion. Achieving this ratio ensures optimal combustion efficiency and minimizes the formation of harmful emissions.