Introduction:
Oxy-acetylene gas welding is a process used to join metal pieces together by melting the base metal and adding a filler material. It involves the combustion of acetylene gas with oxygen to produce a high-temperature flame. To ensure complete combustion and efficient welding, the correct ratio of oxygen to acetylene is crucial.

Explanation:
The stoichiometric ratio of oxygen to acetylene for complete combustion is determined by the chemical equation representing the reaction between the two gases.

The balanced equation for the combustion of acetylene is:
2C2H2 + 5O2 → 4CO2 + 2H2O

From the equation, we can see that 2 moles of acetylene (C2H2) react with 5 moles of oxygen (O2) to produce 4 moles of carbon dioxide (CO2) and 2 moles of water (H2O).

This means that for every 2 moles of acetylene, we need 5 moles of oxygen. To find the volume ratio, we need to consider the molar volume of the gases at the same temperature and pressure.

The molar volume of any ideal gas at standard temperature and pressure (STP) is 22.4 L/mol. Therefore, the volume ratio of oxygen to acetylene can be calculated as follows:

Volume of Oxygen/Volume of Acetylene = (5 moles x 22.4 L/mol) / (2 moles x 22.4 L/mol) = 5/2 = 2.5

Conclusion:
Hence, for complete combustion in oxy-acetylene gas welding, the volume of oxygen required per unit of acetylene is 2.5. This means that for every unit volume of acetylene, 2.5 units of oxygen are needed to ensure efficient and complete combustion.