Problem: The octane rating of gasoline is a relationship of the burning efficiency of the given gasoline mixture to the burning efficiency of octane (C8H18). Like most hydrocarbons, octane reacts with oxygen gas to produce carbon dioxide and water.The unbalanced equation for this reaction is: C8H18 (g) + O2 (g) → CO2 (g) + H2O (g)0.310 mol of octane is allowed to react with 0.870 mol of oxygen.a. How many moles of water are produced in this reaction?b. After the reaction, how much octane is left? 

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We’re being asked to calculate the a) moles of water produced by the reaction, and b) the amount octane remaining.


For this problem, we need to do the following steps:

Step 1: Write and balance a chemical equation for the given reaction.

Step 2: Determine the limiting reactant and calculate the moles of H2O formed.

Step 3: Calculate the mass of octane required for the reaction.

Step 4: Calculate the amount of octane remaining.




Step 1: From the problem, the given combustion reaction is:

C8H18 + O2 → CO2 + H2O


This equation is not yet balanced. To balance it, we have to make sure that the number of elements on both sides is equal.


Balance C: We have 8 C on the reactant side and 1 C on the product side – add a coefficient of 8 to CO2:

C8H18 + O2 → 8 CO2 + H2O


Balance H: We have 18 H on the reactant side and 2 H on the product side – add a coefficient of 9 to H2O:

C8H18 + O2 → 8 CO2 +  9 H2O


Balance O: We have 2 O on the reactant side and 8(2) + 9 = 25 O on the product side – multiply the entire equation by 2 and add a coefficient of 25 to O2:

C8H1825 O2 → 16 CO218 H2O


Now that we have a balanced equation, we can proceed with the problem.


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Problem Details

The octane rating of gasoline is a relationship of the burning efficiency of the given gasoline mixture to the burning efficiency of octane (C8H18). Like most hydrocarbons, octane reacts with oxygen gas to produce carbon dioxide and water.

The unbalanced equation for this reaction is: 

C8H18 (g) + O2 (g) → CO2 (g) + H2O (g)

0.310 mol of octane is allowed to react with 0.870 mol of oxygen.

a. How many moles of water are produced in this reaction?





b. After the reaction, how much octane is left?

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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Limiting Reagent concept. You can view video lessons to learn Limiting Reagent. Or if you need more Limiting Reagent practice, you can also practice Limiting Reagent practice problems.

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Based on our data, we think this problem is relevant for Professor Clevenger's class at USOUTHAL.