Percentage composition, calculation

The percentage composition calculated from the empirical formula closely matches the given data. The formula is reasonable. 

Let s now see how we can use the results of a percentage composition calculation to determine the mass of each element in any amount of the compoimd. Example C.2 presents such a case. 

Calculation of the composition of a mixture on a mole percentage basis requires the use of mole factors, M. These are obtained by dividing the weight factors by the molar masses of each component of the standard solution and normalizing the resulting numbers. A sample calculation utilizing mole correction factors is provided below in the analysis of a mixture of ethanol, heptane, benzene, and ethyl acetate with a GLC equipped with a thermal conductivity detector. The last column shows the percentage composition calculated directly from the measured peak areas, without correction for detector response. The dramatic differences in the calculated composition with and without this correction, as noted in the last two columns, underscore the importance of this correction for quantitative analysis. 

Take some crude cresol mixture (1 g) and dissolve it in cyclohexane (20 mL). Obtain the infrared spectrum for the mixture if necessary, dilute the solution further with cyclohexane to obtain absorbances which will lie on the calibration graphs. From the selected absorption peaks calculate the absorbances for the three individual isomers and use the calibration graphs to calculate the percentage composition of the cresol mixture. 

Mrzss percentage composition is found by calculating the fraction of the total mass contributed by each element present in a compound and expressing the fraction as a percentage. 

Self-TfsT F.4B The mass percentage composition of the compound thionyl difluoride is 18.59% O, 37.25% S, and 44.16% F. Calculate its empirical formula. 

F.l Citral is a fragrant component of lemon oil that is used in colognes. It has the molecular structure shown. Calculate the mass percentage composition of citral (black = C, gray = H, red = O). 

Step 7 Set up a table to calculate the percentage composition of the stream into each unit by copying from the table of component flows. The results are shown in Figure 4.18. Example, from Figure 4.18 ... 

Empirical formulae may also be calculated from percentage composition by mass in a similar way. 

Calculate the percentage composition of the mixture by internal normalization. 

Chemists are often concerned with precisely what percentage of a compound s mass consists of one particular element. Lying awake at night, uttering prayers to Avogadro, they fret over this quantity, called percent composition. Calculating percent composition is trickier than you may think. Consider the following problem, for example. 

From the H NMR spectrum below, which corresponds to a mixture of bromobenzene, dichloromethane and iodoethane, calculate the percentage composition of the three components. The values (without units) given in brackets on the integration curve are proportional to the areas of the corresponding signals. 

These proportions would give when calculated to percentage composition a less efficient mixture than those quoted from the Book of Fires, as shown in the following comparison ... 

A mixture ofN2 and H2 jjas a density ofO 267 g/hter at 700 torr and 30°C por this mixture, calculate (a) the apparent molecular weight, (b) the percentage composition by volume, and (c) the number of molecules in one ml... 

Moisture analyses are important because samples contain water either as chemically combined hydrates or as occluded surface-adsorbed moisture. Water is an inherent part of most biological substances and constitutes >90% of the fresh weight of some plant materials. To afford reproducible analytical results, samples are usually dried before analysis and the percentage composition of the sample is then calculated on a dry basis. 

Calculate the empirical formula of a compound from its mass percentage composition, Self-Test F.3. 

The table shows the values u0 for various percentage compositions of mixtures of CO with air, calculated according to the formula of the theory of flame propagation [5]... 

Inject a 0.1 //I sample of a given pentan-2-one and heptan-3-one mixture, measure the area of each peak and calculate the percentage composition as noted above. 

The approximate molar mass, calculated from the gas density data, is 89 g/mol. The empirical formula, calculated from the percentage composition data, is C2H3O with the empirical formula unit mass of 43.0. The exact molar mass must be (2)(43) = 86.0 g/mol since this is the only multiple of 43.0 (whole-number multiple) reasonably close to the approximate molecular formula of 89 g/mol. The molecule must be the equivalent of 2 empirical formulas CqHgO. 

When 6.027 g of copper(II) chloride is reduced by excess aluminum, 2.851 g of elemental copper is produced according to equation (1). Calculate the weight of chlorine in the sample of CuCl2. Calculate the percentage composition of Cu and Cl in the sample. 

When the hydrochloric acid has cooled to room temperature determine the volume and the specific gravity of the solution in both absorption bottles. Use either a hydrometer or the apparatus shown in Fig. 19. Consult the table on page 371 and determine the percentage composition of the preparation. Calculate the weight of hydrogen chloride obtained, the normality of the solution, and the percentage yield. 

Answer The first step is to calculate the empirical formula. To begin, we need to determine the number of moles of each element. However, the percentage composition does not tell us a mass. To get around this, the commonly used technique is to assume that you have a 100.0-gram sample of the substance. Because 100.0 grams is equal to 100%, you can simply take the percentages and say that the masses are equal to that same amount in grams. From there, the problem proceeds just like the previous example ... 

Examine the following Sample Problem to learn how to calculate the percentage composition of a compound from the mass of the compound and the mass of the elements that make up the compound. Then do the Practice Problems to try expressing the composition of substances as mass percents. 

Calculating Percentage Composition from a Chemical Formula... 

In the previous Practice Problems, you used mass data to calculate percentage composition. This skill is useful for interpreting experimental data when the chemical formula is unknown. Often, however, the percentage composition is calculated from a known chemical formula. This is useful when you are interested in extracting a certain element from a compound. For example, many metals, such as iron and mercury, exist in mineral form. Mercury is most often found in nature as mercury(II) sulfide, HgS. Knowing the percentage composition of HgS helps a metallurgist predict the mass of mercury that can be extracted from a sample of HgS. 

When determining the percentage composition by mass of a homogeneous sample, the size of the sample does not matter. According to the law of definite proportions, there is a fixed proportion of each element in the compound, no matter how much of the compound you have. This means that you can choose a convenient sample size when calculating percentage composition from a formula. 

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