How many moles of steam are produced when 5.74 moles of octane (C8H18) react?

This is a combustion reaction, which always has the hydrocarbon and oxygen as reactants and then carbon dioxide and water (which is steam in this case) as products.

C8H18 + O2 —> CO2 + H2O

Balance your carbons first. You want to get 8 carbons on both sides.

C8H18 + O2 —> 8CO2 + H2O

Now balance your hydrogens. You want 18 hydrogens on both sides.

C8H18 + O2 —> 8CO2 + 9H2O

Now balance your oxygens. You have 2 on the left and 25 on the right. The O2 reactant needs a coefficient of 12.5 to balance this.

C8H18 + 12.5O2 —> 8CO2 + 9H2O

However, you can’t have decimals for coefficients in a reaction. So, multiply everything by 2.

2C8H18 + 25O2 —> 16CO2 + 18H2O

Now, use your mole ratio of 18 mol H2O for every 2 mol C8H18 to solve.

5.74 mol C8H18 • (18 mol H2O / 2 mol C8H18) = 51.7 mol H2O (steam)