Chemical quantities percent yield

The maximum quantity of product that can be obtained from a chemical reaction is the theoretical yield. Invariably some waste occurs during the isolation and purification of products, however no matter how good a chemist you are, you will invariably lose small quantities of material along the way. For this reason, the actual yield of a compound—the quantity of material you actually obtain in the laboratory or chemical plant— is likely to be less than the theoretical yield. The efficiency of a chemical reaction and the techniques used to obtain the desired compound in pure form can be evaluated by calculating the ratio of the actual yield to the theoretical yield. We call the result the percent yield (Figure 4.10). 

Stoichiometry is the quantitative study of products and reactants in chemical reactions. Stoichiometric calculations are best done by expressing both the known and unknown quantities in terms of moles and then converting to other units if necessary. A limiting reagent is the reactant that is present in the smallest stoichiometric amount. It limits the amount of product that can be formed. The amount of product obtained in a reaction (the actual yield) may be less than the maximum possible amount (the theoretical yield). The ratio of the two is expressed as the percent yield. 

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. 

Analyze We are given a chemical equation and the quantity of the limiting reactant (25.0 g of CgHi2). We are asked to calculate the theoretical yield of a product H2CgHg04 and the percent yield if only 33.5 g of product is obtained. 

For a variety of reasons, reactants often yield quantities of products that are less than those calculated from the balanced chemical equation. The quantity calculated from the chemical equation is referred to as the THEORETICAL YIELD, and the PERCENT YIELD is given by... 

When we know the balanced chemical equation for a reaction, we can determine the mole and mass relationships between the reactants and products. Then we use molar masses to calculate the quantities of substances used or produced in a particular reaction. We do much the same thing at home when we use a recipe to make a cake or add the right quantity of water to make soup. In the manufacturing of chemical compounds, side reactions decrease the percent of product obtained. From the actual amount of product, we can determine the percent yield for a reaction. Knowing how to determine the quantitative results of a chemical reaction is essential to chemists, engineers, pharmacists, respiratory therapists, and other scientists and health professionals. 

When we do a chemical reaction in the laboratory, we measure out specific quantities of the reactants. We calculate the theoretical yield for the reaction, which is the amount of product (100%) we would expect if all the reactants were converted to the desired product. When the reaction ends, we collect and measure the mass of the product, which is the actual yield for the product. Because some product is usually lost, the actual yield is less than the theoretical yield. Using the actual yield and the theoretical yield for a product, we can calculate the percent yield. 

Chapter 9, Chemical Quantities in Reactions, describes the mole and mass relationships among the reactants and products and provides calculations of limiting reactants and percent yields. A section on Energy in Chemical Reactions completes the chapter. 

Chemists are interested in assessing the reaction efficiency and, most of the time, use the percent yield of a reaction calculated as the ratio of the acmal yield of a specific product divided by the theoretical yield (based on the limiting reactant). This tool is used to quantify the efficiency of a chemical reaction and to compare the expected product quantity to the actual one. 

The actual peld is the measured quantity of a product obtained in a chemical reaction. (See also theoretical yield and percent yield.)... 

Many-step reactions that have only moderate yields at each step are wasteful and expensive. For this reason, chemists devote much time, effort, and ingenuity in devising reaction sequences and conditions that improve the yields of chemical s mtheses. Our most important industrial chemicals are produced in billion-pound quantities on an annual basis. Here, improving the s thesis yield by even a few tenths of a percent can save a company millions of dollars each year. A good example is fertilizer production, which we describe in our Box. 

From the equation representing the chemical reaction involved, it is evident that 330 g. of silver maleate will theoretically react with 312 g. of ethyl iodide in ethereal solution to produce 172 g. of ethyl maleate. It follows, therefore, that 33 g. (0 1 mol) of silver maleate will react with 31-2 g. (0-2 mol) of ethyl iodide to give a theoretical yield of 17 2 g. (0 1 mol) of ethyl maleate. In practice, the actual yield foimd for these quantities is of the order of 16-0 g. the percentage yield is therefore (16 0/17-2) X 100 = 93 percent. 

The values in parentheses indicate the maximum boundary condition for the quantity under consideration based upon the average historical chemical recovery of 78.42+5.38 percent. Propagation of the listed uncertainties in accordance with established statistical methods yields a relative systematic standard deviation (0 t/R) = 0.037. The frequency distributions for the chemical recoveries exhibited by two chemists are portrayed in Figure 6. The main point to be remembered from these two distributions is that no two individuals will have the same distribution of results. Establishing an a-priori LLD based upon the results of a single individual may not be applicable to other individuals. 

Irradiated fuel may contain up to several percent by mass of fission products, consisting of nearly 200 different isotopes of about 40 different chemical elements whose atomic numbers range from 30 to 66. Nuclides with mass numbers of 85-105 and 130-150 have the highest yields. The precise calculation of the quantities of the various fission products present in fuel at any time during and after irradiation is complicated and best carried out using a computer. A preliminary calculation of the changing... 

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