Stoichiometry conversion factors

Notice how both calculations require you to first convert to moles and then perform a mole-mole conversion using stoichiometry from the reaction equation. Then you convert to the desired units. Both solutions consist of a chain of conversion factors, each factor bringing the units one step closer to those needed in the answer. 

We need to calculate the amount of methyl tert-bu tyl ether that could theoretically be produced from 26.3 g of isobutylene and compare that theoretical amount to the actual amount (32.8 g). As always, stoichiometry problems begin by calculating the molar masses of reactants and products. Coefficients of the balanced equation then tell mole ratios, and molar masses act as conversion factors between moles and masses. 

FIGURE 3.5 A flow diagram summarizing the use of molarity as a conversion factor between moles and volume in stoichiometry calculations. 

For work in the laboratory, it s necessary to weigh reactants rather than just know numbers of moles. Thus, it s necessary to convert between numbers of moles and numbers of grams by using molar mass as the conversion factor. The molar mass of any substance is the amount in grams numerically equal to the substance s molecular or formula mass. Carrying out chemical calculations using these relationships is called stoichiometry. 

The concentration of a substance in solution is usually expressed as molarity (M), defined as the number of moles of a substance (the solute) dissolved per liter of solution. A solution s molarity acts as a conversion factor between solution volume and number of moles of solute, making it possible to carry out stoichiometry calculations on solutions. Often, chemicals are stored as concentrated aqueous solutions that are diluted before use. When carrying out a dilution, only the volume is changed by adding solvent the amount of solute is unchanged. A solution s exact concentration can often be determined by titration. 

Stoichiometry is the series of calculations on the basis of formulas and chemical equations and will be covered in Chapter 4. The use of conversion factors is common even when the relative proportions are not fixed by a chemical formula. Consider a silver alloy used for jewelry production. (Alloys are mixtures of metals and, as mixtures, may be produced in differing ratios of the metals.) A particular alloy contains 86 percent silver. Factors based on this composition, such as... 

The conversion factor problem-solving technique has been used throughout this book, especially in the units on moles and stoichiometry. These problem solutions are generally in a format like this ... 

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. 

The conversion factor for converting between mass and amount in moles is the molar mass of the substance. The molar mass is the sum of atomic masses from the periodic table for the atoms in a substance. Skills Toolkit 3 shows how to use the molar mass of each substance involved in a stoichiometry problem. Notice that the problem is a three-step process. The mass in grams of the given substance is converted into moles. Next, the mole ratio is used to convert into moles of the desired substance. Finally, this amount in moles is converted into grams. 

What conversion factor is present in almost all stoichiometry calculations ... 

What is the key conversion factor needed to solve all stoichiometry problems ... 

Most chemical reactions that occur on the earth s surface, whether in living organisms or among inorganic substances, take place in aqueous solution. Chemical reactions carried out between substances in solution obey the requirements of stoichiometry discussed in Chapter 2, in the sense that the conservation laws embodied in balanced chemical equations are always in force. But here we must apply these requirements in a slightly different way. Instead of a conversion between masses and number of moles, using the molar mass as a conversion factor, the conversion is now between solution volumes and number of moles, with the concentration as the conversion factor. 

Because substances are often found in mixtures, equation stoichiometry problems often include conversions between masses of pure substances and masses of mixtures containing the pure substances, using percentages as conversion factors. See calculations like these at the textbooks Web site. 

To see how molarity can be used in equation stoichiometry problems, let s take a look at the thought process for calculating the number of milliliters of 1.00 M AgN03 necessary to precipitate the phosphate from 25.00 mL of0.500 M Na3P04. The problem asks us to convert from amount of one substance in a chemical reaction to amount of another substance in the reaction, so we know it is an equation stoichiometry problem. The core of our setup will be the conversion factor for changing moles of sodium phosphate to moles of silver nitrate. To construct it, we need to know the molar ratio of AgN03 to Na3P04, which comes from the balanced equation for the reaction. 

Equation stoichiometry problems have at their core the conversion of moles of one substance to moles of another substance. The conversion factor that accomplishes this part of the calculation comes from the coefficients in the balanced equation. Although we have not been given the balanced equation for the combustion of ethanol, we can supply it ourselves by remembering that when a hydrocarbon compound burns completely, all its carbon forms carbon dioxide, and all its hydrogen forms water. 

Between this chapter and Chapter 10, we have now seen three different ways to convert between a measurable property and moles in equation stoichiometry problems. The different paths are summarized in Figure 13.10 in the sample study sheet on the next two pages. For pure liquids and solids, we can convert between mass and moles, using the molar mass as a conversion factor. For gases, we can convert between volume of gas and moles using the methods described above. For solutions, molarity provides a conversion factor that enables us to convert between moles of solute and volume of solution. Equation stoichiometry problems can contain any combination of two of these conversions, such as we see in Example 13.8. 

Convert between volume of gas and moles of gas in an equation stoichiometry problem using either the molar volume at STP, the ideal gas equation, or i as a conversion factor. 

Analytical chemistry deals with measurements of solids and solution concentrations of them, from which we calculate masses. Thus, we prepare solutions of known concentrations that can be used to calibrate instruments or to titrate sample solutions. We calculate the mass of analyte in a solution from its concentration and the volume. We calculate the mass of product expected from the mass of reactants. All of these calculations require a knowledge of stoichiometry, that is, the ratios in which chemicals react, from which we apply appropriate conversion factors to arrive at the desired calculated results. 

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. 

When reactions occur in solution, reactant and product amounts are given in terms of concentration and voiume. Moiarity is the number of moles of solute dissolved in one liter of solution. Using molarity as a conversion factor, we apply the principles of stoichiometry to all aspects of reactions in solution. 

Quantitative acid hydrolysis with 72% (w/w) H2SO4 [19] was used to characterize the BSG feedstock. The monosaccharides and acetic acid were determined by HPLC to estimate (after corrections for stoichiometry and sugar decomposition) the contents of glucan (cellulose) and hemicelluloses (xylan, arabinan, and acetyl groups) in the sample. The acid-insoluble residue after hydrolysis was recovered by filtration and considered as Klason lignin after correction for the acid-insoluble ash. Protein was determined by the KJeldahl method [20] using the Ax 6.25 conversion factor. 

Ans. The stoichiometry tells us that for every 2 mol of O3 that appears, 3 mol of O2 must disappear. Using that information, create a conversion factor of the proper units ... 

The dramatic influence that these two stoichiometry and "conversion factors can have on molecular weight cannot be overemphasized, and can be explained in terms of the classic theory of polycondensation reactions developed by Carothers [7, 40]. Based on this theory, the number average degree of polymerization DP (i.e. the value of M divided by the molecular weight of a repeat unit) is given by the expression in Eq. 1.9. 

Just like unit conversion factors, these mole ratios can be inverted as needed to carry out a particular calculation. Example Problem 4.1 shows how to use mole ratios in a stoichiometry problem. 

The heart of any stoichiometry problem is the balanced chemical equation that provides the mole ratio we need. We must convert between masses and the number of moles in order to use this ratio, first for the reactant, 5, and at the end of the problem for the product, P4S3. Molar mass is used to provide the needed conversion factors. The phrase, excess of phosphorus, tells us that we have more than enough P4 to consume 153 g of 8 completely. 

You should recognize this as a reaction stoichiometry problem because it is asking us how much CaC03 will be produced. As for any stoichiometry problem, we should write a balanced chemical equation for the reaction. Then convert the volume of gas into moles and proceed as usual. Because the gas volume given is at STP, we can use the molar volume we calculated above as a conversion factor. 

We first use the conversion factor of 1 mol/22.4 L to convert the liters H2 to moles. Then we use our Problem-Solving Strategy for Stoichiometry Problems to find the moles of aluminum produced ... 

In a balanced equation, the number of moles of one substance is stoichiometri-cally equivalent to the number of moles of any other substance. The term stoi-chiometrically equivalent means that a definite amount of one substance is formed from, produces, or reacts with a definite amount of the other. These quantitative relationships are expressed as stoichiometricolly equivalent molar ratios that we use as conversion factors to calculate these amounts. Table 3.3 presents the quantitative information contained in the equation for the combustion of propane, a hydrocarbon fuel used in cooking and water heating ... 

By using molarity as a conversion factor, we can apply the principles of stoichiometry to reactions in solution. 

Reaction Stoichiometry Chemical Equations as Conversion Factors... 

Conversion Factors from a Chemical Equation Mass-Mass Stoichiometry Percent Yield Limiting Reactants ... 

Stoichiometry problems can become long most unit paths have three steps and some have four or more. But the problems are not difficult if you recognize the unit path, know the conversion factor for each step, and then apply the factors correctly. That s the skill you should develop in this chapter. 

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