what mass is required to burn 688 g of octane, C8H18, completely?

Molar mass of C₈H₁₈ = 12×8 + 1×18 = 114 g/mol
Molar mass of O₂ = 16×2 = 32 g/mol

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
2 moles of C₈H₁₈ need 25 moles of O₂ for complete combustion.

Number of moles of C₈H₁₈ = (688 g) / (114 g/mol) = 6.035 mol
Number of moles of O₂ needed = (6.035 mol) × 25/2 = 75.44 mol
Mass of O₂ needed = (75.44 mol) × (32 g/mol) = 2414 g

Are you asking what mass of oxygen is required to burn 688 g of octane completely? If so, convert the 688 g of octane into moles of octane by dividing by the molecular weigh octane. Next write a balanced equation for the combustion of octane w Oxygen, O2. Next note the ratio of moles of oxygen / moles of octane and then multiply this ratio by the moles of oxygen. This gives you the moles of oxygen needed. Next convert the moles of oxygen needed to grams. I do not do your homework but will show you how which I just did.