Stoichiometry mole conversions

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. 

ENB content (wt.%) Mooney viscosity ML(1+4)125°C sulfur content (phr) ENB conversion (%) converted ENB (mole/kg EPDM) molecular weight between crosslinks (kg/mole) crosslink density (mole/kg) crosslink stoichiometry (mole/mole)... 

Figure 10.1 Mole Conversions for Stoichiometry Problems The double-headed arrows indicate that the conversions can be made in either direction.

Figure 10.2 Mass and Mole Conversions for Stoichiometry Problems...

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. 

Now you are ready to try your first complete stoichiometry problems, where you quantitatively analyze chemical reactions. The mass-mass problem is where you either know the mass of the product that you want to produce and calculate the mass of the reactant(s) you start with, or you know the mass of the reactant(s) you start with and calculate the mass of the product you will end up with. As with mole conversion problems, there are a variety of these types of problems, with a varying range of difficulty. We will start off with some of the easier types and work our way up to harder problems. 

The second step is the typical equation stoichiometry mole-to-mole conversion, using the coefficients from the balanced equation ... 

The remaining OH flux, (t)OHb, can be assigned to production of Alb. The concentration of Alb is known from the analyses of the solutions. If the ratio CoHb/CAib has a value of 2.0, the stoichiometry for conversion of Ala to Alb can be represented directly by equation 2, and one mole of added OHb produces one mole of A1 in the form of Alb. If the OHb/Alb ratio is greater or less than 2.0 the rate of OH addition requires a further correction to give a rate constant in terms of aluminum that is reacting to form Alb. This correction is designated n in the final rate equation, which is written ... 

Chemical stoichiometry is the area of study that considers the quantities of materials in chemical formulas and equations. Quite simply, it is chemical arithmetic. The word itself is derived from stoicheion, the Greek word for element and metron, the Greek word for measure. When based on chemical formulas, stoichiometry is used to convert between mass and moles, to calculate the number of atoms, to calculate percent composition, and to interpret the mole ratios expressed in a chemical formula. Most topics in chemical arithmetic depend on the interpretation of balanced chemical equations. Mass/mole conversions, calculation of limiting reagent and percent yield, and various relationships among reactants and products are commonly included in this topic area. 

The stoichiometry of conversion of /3-carotene to retinol is still an unsettled issue. Central cleavage of /3-carotene theoretically yields 2 mol retinol per mole /3-carotene (Goodman and Huang, 1%5 Olson and Hayaishi, 1965 Olson, 1989) and eccentric cleavage of /3-carotene yields 1 mol retinol per mole /3-carotene (Olson, 1989, Krinsky et al., 1994). Brubacher and Weiser (1985) determined the retinol equivalent of /3-carotene in vivo using rats and chicks and found that 1 mol of absorbed /3-carotene yielded 1 mol retinol. Because the body reserves of retinol in these animals were low, the yield of retinol from /3-carotene was probably maximal. Based on these in vivo results, a ratio of 1 mol retinol per mole /3-carotene (after absorption) was used in constructing the present compartmental model of the dynamics of /3-carotene metabolism. 

Corollary In warm-up examples 4 and 5 (chemical reactors), we had information about stoichiometry and conversion, and the proposed procedure was to construct a table to take into account the moles entering the reactor, moles reacting, and the moles leaving the reactors (reagents and products). This is a convenient procedure and facilitates the material balance in the reactor. On the other hand, in the bioreactor problems, we had information about the disappearance of the substrate (kinetics), and in that case it was easier just to formulate the mass balance like (8.6). 

Take a moment to think through the logic of the calculation sequence in stoichiometry. Look particularly at the mole-to-mole conversion in the middle. If you understand the process, apart from a specific problem, you will be able to recognize and solve other kinds of stoichiometry problems in chapters to come. 

Thermochemical stoichiometry problems have one less step than other stoichiometry problems because they involve only one substance. There is no mole-to-mole conversion, but rather a mole-to-energy change between the single substance and the AH of the reaction. Watch the sign of AH if the wording of the problem is such that it must be taken into account. 

Such problems as this one involve many steps or conversions. Try to break the problem into simpler ones involving fewer steps or conversions. It may also help to remember that solving a stoichiometry problem involves three steps (1) converting to moles, (2) converting between moles, and (3) converting from moles. Use molarities and molar masses to carry out volume-mole conversions and gram-mole conversions, respectively, and stoichiometric factors to carry out mole-mole conversions. The stoichiometric factors are constructed from a balanced chemical equation. 

Since the volume depends on conversion or time in a constant pressure batch reactor, consider the mole balance in relation to the fractional conversion X. From the stoichiometry. 

Specifically, it has recently been found 149) that diarylthallium tri-fluoroacetates may be converted into aromatic iodides by refluxing a solution in benzene with an excess of molecular iodine. Yields are excellent (74-94%) and the overall conversion represents, in effect, a procedure for the conversion of aromatic chlorides or bromides into aromatic iodides via intermediate Grignard reagents. The overall stoichiometry for this conversion is represented in Eq. (10), and it would appear that the initial reaction is probably formation of 1 mole of aromatic iodide and 1 mole of arylthallium trifluoroacetate iodide [Eq. (8)] which subsequently spontaneously decomposes to give a second mole of aromatic iodide and thallium(I) trifluoroacetate [Eq. (9)]. Support for this interpretation comes from the... 

It can be seen in Table 6.9 that as pressure increases, nonideal behavior changes the equilibrium conversion. Note that like Ka, K also depends on the specification of the number of moles in the stoichiometric equation. For example, if the stoichiometry is written as ... 

In a stoichiometry problem, (a) if the mass of a reactant is given, what conversions (if any) should be made (b) If a number of molecules is given, what conversions (if any) should be made (c) If a number of moles is given, what conversions (if any) should be made ... 

An exothermic reaction with the stoichiometry A 2B takes place in organic solution. It is to be carried out in a cascade of two CSTR s in series. In order to equalize the heat load on each of the reactors it will be necessary to operate them at different temperatures. The reaction rates in each reactor will be the same, however. In order to minimize solvent losses by evaporation it will be necessary to operate the second reactor at 120 °C where the reaction rate constant is equal to 1.5 m3/kmole-ksec. If the effluent from the second reactor corresponds to 90% conversion and if the molal feed rate to the cascade is equal to 28 moles/ksec when the feed concentration is equal to 1.0 kmole/m3, how large must the reactors be If the activation energy for the reaction is 84 kJ/mole, at what temperature should the first reactor be operated ... 

A useful tool for dealing with reaction stoichiometry in chemical kinetics is a stoichiometric table. This is a spreadsheet device to account for changes in the amounts of species reacted for a basis amount of a closed system. It is also a systematic method of expressing the moles, or molar concentrations, or (in some cases) partial pressures of reactants and products, for a given reaction (or set of reactions) at any time or position, in terms of initial concentrations and fractional conversion. Its use is illustrated for a simple system in the following example. 

This is a critical chapter in your study of chemistry. Our goal is to help you master the mole concept. You will learn about balancing equations and the mole/mass relationships (stoichiometry) inherent in these balanced equations. You will learn, given amounts of reactants, how to determine which one limits the amount of product formed. You will also learn how to determine the empirical and molecular formulas of compounds. All of these will depend on the mole concept. Make sure that you can use your calculator correctly. If you are unsure about setting up problems, refer back to Chapter 1 of this book and go through Section 1-4, on using the Unit Conversion Method. Review how to find atomic masses on the periodic table. Practice, Practice, Practice. 

This balanced equation can be read as 4 iron atoms react with 3 oxygen molecules to produce 2 iron(III) oxide units. However, the coefficients can stand not only for the number of atoms or molecules (microscopic level) but they can also stand for the number of moles of reactants or products. So the equation can also be read as 4 mol of iron react with 3 mol of oxygen to produce 2 mol ofiron(III) oxide. In addition, if we know the number of moles, the number of grams or molecules may be calculated. This is stoichiometry, the calculation of the amount (mass, moles, particles) of one substance in the chemical equation from another. The coefficients in the balanced chemical equation define the mathematical relationship between the reactants and products and allow the conversion from moles of one chemical species in the reaction to another. 

In the absence of DMSO the conversion of diphenylmethyl hydroperoxide to benzophenone apparently does follow Reaction 6, at least in alcohol-containing solvents. The stoichiometry becomes nearly one mole of oxygen per mole of diphenylmethane, and the carbinol is eliminated as an intermediate. Table I lists the observed stoichiometries and initial rates of oxidation of diphenylmethane. In pyridine-, DMF-, or HMPA-containing solvents, a high yield of benzophenone was isolated upon hydrolysis after oxygen absorption had ceased. In pure HMPA there was considerable evolution of oxygen upon hydrolysis. 

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. 

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