Stoichiometric calculation

The solution to every stoichiometric problem requires a balanced chemical equation. 

Real-World Reading Link Baking requires accurate measurements. That is why it is necessary to follow a recipe when baking cookies from scratch. If you need to make more cookies than a recipe yields, what must you do  

From the balanced equation, you know that two moles of potassium yields one mole of hydrogen. But how much hydrogen is produced if only 0.0400 mol of potassium is used To answer this question, identify the given, or known, substance and the substance that you need to determine. The given substance is 0.0400 mol of potassium. The unknown is the number of moles of hydrogen. Because the given substance is in moles and the unknown substance to be determined is also in moles, this problem involves a mole-to-mole conversion. 

To solve the problem, you need to know how the unknown moles of hydrogen are related to the known moles of potassium. In Section 11.1, you learned to derive mole ratios from the balanced chemical equation. Mole ratios are used as conversion factors to convert the known number of moles of one substance to the unknown number of moles of another substance in the same reaction. Several mole ratios can be written from the equation, but how do you choose the correct one  

The following Example Problems show mole-to-mole, mole-to-mass, and mass-to-mass stoichiometry problems. The process used to solve these problems is outlined in the Problem-Solving Strategy below. 

There are three basic stoichiometric calculations mole-to-mole conversions, mole-to-mass conversions, and mass-to-mass conversions. All stoichiometric calculations begin with a balanced equation and mole ratios. 

use the mole ratio to convert the known number of moles of chlorine to the number of moles of table salt. Use the formula below. 

A piece of magnesium burns in the presence of oxygen, forming magnesium oxide (MgO). How many moles of oxygen are needed to produce 12 moles of magnesium oxide  

Write the balanced equation and the mole ratio that relates mol O2 to mol MgO. 

Multiply the known number of moles of MgO by the mole ratio. 

Potassium metal reacts vigorously with water, releasing so much heat that the hydrogen gas formed in the reaction catches fire. 

Suppose a chemist needs to obtain a certain amount of product from a reaction. How much reactant must be used Or, suppose the chemist wants to know how much product will form if a certain amount of reactant is used. Chemists use stoichiometric calculations to answer these questions. 

Recall that stoichiometry is the study of quantitative relationships between the amounts of reactants used and the amounts of products formed by a chemical reaction. What are the tools needed for stoichiometric calculations All stoichiometric calculations begin with a balanced chemical equation, which indicates relative amounts of the substances that react and the products that form. Mole ratios based on the balanced chemical equation are also needed. You learned to write mole ratios in Section 12.1. Finally, mass-to-mole conversions similar to those you learned about in Chapter 11 are required. 

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As shown in the following examples, the application of conservation principles simplifies stoichiometric calculations. 

Quantitative Calculations In acid-base titrimetry the quantitative relationship between the analyte and the titrant is determined by the stoichiometry of the relevant reactions. As outlined in Section 2C, stoichiometric calculations may be simplified by focusing on appropriate conservation principles. In an acid-base reaction the number of protons transferred between the acid and base is conserved thus... 

These types of stoichiometric calculations are commonplace and can provide reliable estimates for the material balance. As with any calculation method, one should list the assumptions to qualify the accuracy of the estimate. Limited field measurements can always be done later to verify the estimated emissions. 

The emphasis is on writing and balancing chemical equations for these reactions. All of these reactions involve ions in solution. The corresponding equations are given a special name net ionic equations. They can be used to do stoichiometric calculations similar to those discussed in Chapter 3. 

Stoichiometric calculations for redox reactions in water solution are carried out in much the same way as those for precipitation reactions (Example 4.5) or acid-base reactions (Example 4.7). 

Stoichiometric calculations are based upon two assumptions. First, we assume that only a single reaction need be considered to describe the chemical changes occurring. Second, we assume that the reaction is complete. For example, consider the question, How much iron is produced per mole of Fe203 reacted with aluminum in the following reaction ... 

If these assumptions are valid, however, stoichiometric calculations provide a reliable basis for quantitative predictions. It is important to be able to make these calculations with ease. Fortunately, they all can be made with a single pattern based upon the mole concept. 

The equation for a chemical reaction speaks in terms of molecules or of moles. It contains the basis for stoichiometric calculations. However, in the laboratory a chemist measures amounts in such units as grams and milliliters. The first step in any quantitative calculation, then, is to convert the measured amounts to moles. In mole units, the balanced reaction connects quantities of reactants and products. Finally, the result is expressed in the desired units (which may not necessarily be the same as the original units). 

Stoichiometric coefficients are exact numbers so they do not limit the significant figures of stoichiometric calculations (see Appendix 1C). 

Carry out stoichiometric calculations for any two species taking part in a chemical reaction (Toolbox L.l and Example L.l). 

Stoichiometric calculations of the amount of product formed in a reaction are based on an ideal view of the world. They suppose, for instance, that all the reactants react exactly as described in the chemical equation. In practice, that might not be so. Some of the starting materials may be consumed in a competing reaction, a reaction taking place at the same time as the one in which we are interested and using some of the same reactants. Another possibility is that the reaction might not be complete at the time we make our measurements. A third possibility—of major importance in chemistry and covered in several chapters of this text—is that many reactions do not go to completion. They appear to stop once a certain proportion of the reactants has been consumed. 

The heat absorbed or given off by a reaction can be treated like a reactant or product in a stoichiometric calculation. 

Stoichiometric calculations always require amounts in moles. For gases, amounts In moles are usually calculated from the ideal gas equation. Example shows how to do this. 

Any of the types of problems discussed in Chapters 3 and 4 can involve gases. The strategy for doing stoichiometric calculations is the same whether the species involved are solids, liquids, or gases. In this chapter, we add the ideal gas equation to our equations for converting measured quantities into moles. Example is a limiting reactant problem that involves a gas. 

These relationships provide complete stoichiometric information regarding the equilibrium. Just as amounts tables are usetiil in doing stoichiometric calculations, a concentration table that provides initial concentrations, changes in concentrations, and equilibrium concentrations is an excellent way to organize Step 5 of the problem-solving... 

The net ionic equation, like all balanced chemical equations, gives the ratio of moles of each substance to moles of each of the others. It does not immediately yield information about the mass of the entire salt, however. (One cannot weigh out only Ba2+ ions.) Therefore, when masses of reactants are required, the specific compound used must be included in the calculation. The use of net ionic equations in stoichiometric calculations will be more important after study of molarity (Chap. 10). 

You know that a balanced equation represents relationships between the quantities of reactants and products. For a reaction that takes place in a cell, stoichiometric calculations can also include the quantity of electricity produced or consumed. Stoichiometric calculations in electrochemistry make use of a familiar unit—the mole. 

For stoichiometric calculations, you also need to know the electric charge on a mole of electrons. This charge can he calculated by multiplying the charge on one electron and the number of electrons in one mole (Avogadro s number). The charge on a mole of electrons is known as one faraday (IF), named after Michael Faraday. 

Perform a sample stoichiometric calculation involving the quantity of electricity used in electrolytic processes. 

These calculations are so basic to the field that you should go back and carefully review the two examples in this section the calculation of weight percents in N2O and the inference of the formula for aluminum oxide. Then you can practice stoichiometric calculations on the following pair of problems, which are answered and explained in Appendix A. Many such practice exercises are included in this book so you can determine whether you understand the major concepts of chemistry. It is well worth your time to study these examples and their explanations in Appendix A until you can do the calculations correctly. 

UPD of various metal ions is accompanied by the adsorption of anions. Evidently, such additional adsorption makes stoichiometric calculations based on the determined charge value misleading. 

A concise synthesis of branched epoxy resins is presented. Stoichiometric calculations are discussed which treat the synthesis in an idealized statistical model. The calculations can be adapted to any well characterized reactants. 

A chemical formula tells the numbers and the kinds of atoms that make up a molecule of a compound. Because each atom is an entity with a characteristic mass, a formula also provides a means for computing the relative weights of each kind of atom in a compound. Calculations based on the numbers and masses of atoms in a compound, or the numbers and masses of molecules participating in a reaction, are designated stoichiometric calculations. These weight relationships are important because, although we may think of atoms and molecules in terms of their interactions as structural units, we often must deal with them in the lab in terms of their masses—with the analytical balance. In this chapter, we consider the Stoichiometry of chemical formulas. In following chapters, we look at the stoichiometric relations involved in reactions and in solutions. 

The masses of the atoms are the basis for all stoichiometric calculations. Long before the actual masses of atoms were known, a relathe scale of masses, called the atomic ueightscale, was devised. Since 1961, this relative scale has been based on the assignment of the value 12.00000 to the most common isotope of carbon. The table on the inside back cover lists approximate values of the... 

All of the important stoichiometric calculations that relate the weights and volumes of starting materials to the weights and volumes of products typically involve just three simple steps. 

When two or more substances are mixed together in a manner that is homogeneous and uniform at the molecular level, the mixture is called a solution. The component (usually a liquid) that is present in much larger quantity than the others is called the solvent the other components are the solutes. The concentration of a solution describes the amount of solute present in a given amount of solution. When a solution is involved in a reaction, the stoichiometric calculations must take into account two quantities not previously discussed the concentration of the solution, and its volume. 

After we have developed an ionic equation for an electron-transfer reaction, we frequently need to show the molecules involved in the solutions—that is, the substances that are initially put into the solution, and those that are obtained from it after the reaction has occurred. We must have such molecular equations if stoichiometric calculations are to be made. 

The stoichiometric calculations of Chapters 12 and 13 are based on the mole as the fundamental chemical unit in reactions. An alternative method of calculation utilizes the equivalent as a fundamental chemical unit. There are two kinds of equivalents, the type depending on the reaction in question we shall refer to them as acid-base equivalents (or simply as equivalents) and electron-transfer equivalents (or E-T equivalents). The concept of an equivalent is particularly useful when dealing with complex or unknown mixtures, or when working out the structure and properties of unknown compounds. In addition, it emphasizes a basic characteristic of all chemical reactions that is directly applicable to all types of titration analyses. 

Sulfur is one of the few exceptions to the constancy of isotopic proportions, in that there is sufficient variation, dependent upon the source of the sulfur, to cause a variation in its atomic mass by approximately 0.01%. For normal stoichiometric calculations, however, this small variation is unimportant. 

Since the element of time is usually involved as die basis of a stoichiometric calculation, proper quantitative deductions often depend on adequate knowledge of other laws or principles, such as those governing rates of reaction and those pertaining to chemical equilibria. When materials in die gaseous state are involved, die general gas laws are of great utility. 

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