We have the ethanol combustion reaction to produce CO2 and water. The balanced equation of the reaction is:

We must first find the moles of ethanol corresponding to 421 grams. To do this we divide the mass by the molar mass of ethanol. The molar mass of ethanol is: 46.07g/mol. The moles of ethanol will be:

Now, by the stoichiometry of the reaction, we see that the Oxygen to Ethanol ratio is 9/4. So, the moles of oxygen needed will be:

We find the grams of oxygen by multiplying the moles by the molar mass. The molar mass of O2 is 31.9988g/mol.

Answer: To complete the combustion of 421 g of ethanol are needed 658 grams of oxygen