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# Stoichiometry and Chemical Equations Study Guide (page 2)

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Updated on Sep 24, 2011

## Reaction Stoichiometry

Stoichiometry establishes the quantities of reactants (used) and products (obtained) based on a balanced chemical equation. With a balanced equation, you can compare reactants and products, and determine the amount of products that might be formed or the amount or reactants needed to produce a certain amount of a product. However, when comparing different compounds in a reaction, you must always compare in moles (i.e., the coefficients). The different types of stoichiometric calculations are summarized in Figure 5.1.

#### Example:

How many grams of water are produced when 24.3 g of methane is reacted with excess oxygen?

 1. Write the unbalanced equation: CH4 + O2 → CO2 + H2O 2. Balance the equation: CH4 + 2O2 → CO2 + 2H2O 3. Identify the amounts given and needed: CH4 + 2O2 → CO2 + 2H2O 24.3 g → ? 4. Use the molar masses and mole ratios (coeffi-cients) to set up the calculation: = 54.6 g H2O Glucose (C6H12O6) ferments over time to produce ethanol (C2H5OH) and carbon dioxide. How many grams of glucose are needed to produce 100.0 g of ethanol? 5. Write the unbalanced equation: C6H12O6 → C2H5OH + CO2 6. Balance the equation: C6H12O6 → 2C2H5OH + 2CO2 7. Identify the amounts given and needed: C6H12O6 → 2C2H5OH + 2CO2 ? → 100.0 g 8. Use the molar masses and mole ratios (coeffi-cients) to set up the calculation:

## Limiting Reactant

The reagent that is consumed first in a reaction is called the limiting reactant or reagent. In previous examples and problems, the assumption was that one reactant was in excess and the other was the limiting reactant. However, if a known amount of each reactant is added to a reaction vessel, then the limiting reactant must be calculated. Because the limiting reactant is consumed and limits the amount of products being formed, the easiest method of finding the limiting reactant is to simply calculate the amount of product that could be formed from each reactant. The reactant that produces the least amount of product is the limiting reagent. The amount of product that the limiting reactant can produce is called the theoretical yield of the reaction.

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