First find numbers of moles of Al and O₂, these are the actual moles.

Second, divide the equation by 4 to find the number of moles of O₂ required to react with one mole of Al – this is the normalized equation.

Multiply the actual moles for Al by the coefficient for O₂ in the normalized equation, this gives the theoretical moles for O₂. Then find the excess reactant by comparing the actual moles of O₂ with the theoretical moles of O₂.
The excess may then be calculated in moles (actual-theoretical).
Next, multiply the excess number of moles by the molar mass of the excess reactant to find the excess amount in grams.