Calculations Involving a Limiting Reactant

In Example 9.6 we answered one of the questions we posed in the introduction to this chapter. Now let s see if you can answer the other question posed there. Determine what mass of carbon monoxide and what mass of hydrogen are required to form 6.0 kg of methanol by the reaction 

AIMS To learn to recognize the limiting reactant in a reaction. To learn to use the limiting reactant to do stoichiometric calculations. 

Farmer Rodney Donala looks out over his corn fields in front of his 30,000-gallon tank (at right) of anhydrous ammonia, a liquid fertilizer. 

Let s consider the question. What mass of water is required to react exactly with 249 g of methane That is, how much water will just use up all of the 249 g of methane, leaving no methane or water remaining  

This problem requires the same strategies we developed in the previous section. Again, drawing a map of the problem is helpful. 

The hydrogen for this process is produced from the reaction of methane with water  

however, 2.50 X 103 kilograms of methane is mixed with 3.00 X 103 kilograms of water, the methane will be consumed before the water runs out. The water will be in excess. In this case the quantity of products formed will be determined by the quantity of methane present. Once the methane is consumed, no more products can be formed, even though some water still remains. In this situation, because the amount of methane limits the amount of products that can be formed, it is called the limiting reactant, or limiting reagent. In any stoichiometry problem it is essential to determine which reactant is the limiting one to calculate correctly the amounts of products that will be formed. 

To further explore the idea of a limiting reactant, consider the ammonia synthesis reaction  

Assume that 5 N2 molecules and 9 H2 molecules are placed in a flask. Is this a stoichiometric mixture of reactants, or will one of them be consumed before the other runs out From the balanced equation we know that each N2 molecule requires 3 H2 molecules for the reaction to occur  

Since the actual ratio (1.8 1) of H2/N2 is less than the ratio required by the balanced equation (3 1), there is not enough hydrogen to react with all 

Manufacturers of cars, bicycles, and appliances order parts in the same proportion as they are used in their products. For example, auto manufacturers order four times as many wheels as engines and bicycle manufacturers order twice as many pedals as seats. Likewise, when chemicals are mixed together so that they can undergo a reaction, they are often mixed in stoichiometric quantities—that is, in exactly the correct amounts so that all 

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IgQllllailMr Module 8b Stoichiometry and Limiting Reactants (Pt. 2) covers concepts in this section. 

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Calculations Involving a Limiting Reactant 73 Discussion Questions and Exercises 79... 

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Calculations Involving a Limiting Reactant 106 For Review 113 Key Terms 113 Questions and Exercises 115... 

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