Solving Stoichiometry Problems

Solving stoichiometry problems always requires finding the number of moles of the first reactant, using the coefficients of the balanced equation to find the number of moles of the second reactant, and then finding the amount of the second reactant. The flow diagram in Figure 3.5 summarizes the situation. 

Assume that your friend has missed several chemistry classes and that she has asked you to help her prepare for a stoichiometry test. Unfortunately, because of other commitments, you do not have time to meet face to face. You agree to email your friend a set of point-form instructions on how to solve stoichiometry problems, including those that involve a limiting reactant. She also needs to understand the concept of percentage yield. Write the text of this email. Assume that your friend has a good understanding of the mole concept. 

Recall that stoichiometry involves calculating the amounts of reactants and products in chemical reactions. If you know the atoms or ions in a formula or a reaction, you can use stoichiometry to determine the amounts of these atoms or ions that react. Solving stoichiometry problems in solution chemistry involves the same strategies you learned in Unit 2. Calculations involving solutions sometimes require a few additional steps, however. For example, if a precipitate forms, the net ionic equation may be easier to use than the chemical equation. Also, some problems may require you to calculate the amount of a reactant, given the volume and concentration of the solution. 

Review the method for solving stoichiometry problems you learned in Chapter 7,... 

If necessary, review Chapter 6 to solve stoichiometry problems that give the names of compounds, rather than their formulas. 

I Include the formulas of the substances involved with the units when applying the factor label method to solve stoichiometry problems. For example, write g NaCl or mol NaCl, rather than just g or mol. ... 

Explain why mole ratios are central to solving stoichiometry problems. 

Solve stoichiometry problems involving mass by using molar mass. 

Solve stoichiometry problems involving the volume of a substance by using density. 

Substances are usually measured by mass or volume. As a result, before using the mole ratio you will often need to convert between the units for mass and volume and the unit mol Yet each stoichiometry problem has the step in which moles of one substance are converted into moles of a second substance using the mole ratio from the balanced chemical equation. Follow the steps in Skills Toolkit 2 to understand the process of solving stoichiometry problems. 

The thought process in solving stoichiometry problems can be broken down into three basic steps. First, change the units you are given into moles. Second, use the mole ratio to determine moles of the desired substance. Third, change out of moles to whatever unit you need for your final answer. And if you are given moles in the problem or need moles as an answer, just skip the first step or the last step As you continue reading, you will be reminded of the conversion factors that involve moles. 

Think through the three basic steps used to solve stoichiometry problems change to moles, use the mole ratio, and change out of moles. Know which conversion factors you will use in each step. 

Why do you need to use amount in moles to solve stoichiometry problems Why can t you just convert from mass to mass ... 

Why is it necessary to use mole ratios in solving stoichiometry problems ... 

Whenever you can relate the given information to moles, you can solve stoichiometry problems. In this case, the ideal gas law is the bridge that you need to get from moles to the answer. 

In Chapter 3 we used relationships between amounts (in moles) and masses (in grams) of reactants and products to solve stoichiometry problems. When the reactants and/or products are gases, we can also use the relationships between amounts (moles, ri) and volume (V) to solve such problems (Figure 5.12). The following examples show how the gas laws are used in these calculations. 

Comment This problem highlights a key point for solving stoichiometry problems convert the information given into moles. Then, use the appropriate molar ratio and any other conversion factors to complete the solution. 

Solving stoichiometry problems for reactions in solution requires the same approach as before, with the additional step of converting the volume of reactant or product to moles (1) balance the equation, (2) find the number of moles of one substance, (3) relate it to the stoichiometrically equivalent number of moles of another substance, and (4) convert to the desired units. 

By converting the variables P, V, and T of gaseous reactants (or products) to amount (n, mol), we can solve stoichiometry problems for gaseous reactions. 

When solving stoichiometry problems for solution reactions, what type of chemical equation is most convenient to use ... 

The key to solving stoichiometry problems is the mole. How do we find the number of moles when solutions are mixed to produce a reaction ... 

We need to use the strategy for solving stoichiometry problems that we learned in Chapter 3. 

Properties of the gaseous state predicted by the ideal gas law are within 5% for gases under ordinary conditions. In other words, given a set of conditions, we can predict or calculate the properties of a gas to be within 5% by applying the ideal gas law. Here we will review several important concepts that are helpful for solving Stoichiometry Problems Involving Gases. 

The following list summarizes the steps to take in solving stoichiometry problems in which the amounts of two (or more) reactants are given. 

Steps for Solving Stoichiometry Problems Involving Limiting Reactants... 

The balanced chemical equation provides mole ratios for all reactants and products. To use these ratios, usually we have to convert information from something that can be easily measured—such as the mass or volume of a reactant or product—to moles. This fact establishes the main pattern for solving stoichiometry problems. First, we must look at the information we know about a reactant or... 

The general approach for solving stoichiometry problems is summarized next. Write a balanced equation for the reaction. 

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