Experimental data, percent composition

To show you that all this really does work, I ve listed the experimental composition data for the isopropyl/isobutyl alcohol system from Landolt-Bornstein (Landolt-Bomstein is to physical chemistry what Beilstein is to organic. And wouldn t that make for a wild analogy question on the college board entrance exams ), along with my calculated data (Table 2) (That explains my choice of temperatures for Table 1.). I ve also given the absolute and percent differences between the experimental data, and what I ve calculated. These differences are on the order of 1% or less, a very good agreement, indeed. 

When a combustible substance is mixed with air, the mixture will explode only when it is neither too rich nor too lean. The lower explosion limit (LEL) is the minimum volume percent of the substance in air with flammability, which is separated from the upper explosion limit (UEL) by the explosive concentration range. The tabulations in handbooks are based on experimental data, and sometimes derived from estimation methods based on the elemental composition of the fuel as CmEtxOy. Figure 6.11 shows the LEL for the series of normal paraffins and of 1-alcohols versus the number of carbon atoms. There are two ways to plot the results, which show that, for paraffins, the volume percent shows a steeply declining trend, but the weight percent shows a mildly increasing trend. One may conclude that a smaller volume percent of higher paraffin... 

Because the original percent composition data is typically experimental, expect to see a bit of error in the numbers. For example, 2.03 is probably within experimental error of 2, 2.99 is probably 3, and so on. 

Alcohol is to be extracted from an aqueous solution by pure ether in an extraction column. The alcohol solution, containing 30% alcohol by weight, enters the top of the column at a rate of 370 kg/h. The ether is to be fed to the column bottom at 350 kg/h. About 90 percent of the alcohol is to be extracted that is, alcohol concentration in the exiting aqueous stream should be about 3%. Experimental data on the compositions of pairs of water-rich and ether-rich phases in equilibrium are given in Table 9.1. Calculate the flow rates and compositions of the exiting raffinate (i.e., alcohol-depleted, aqueous) phase and the extract (i.e., alcohol-enriched, ether) phase. Also calculate the number of extraction stages needed. 

Formulas describe the composition of compounds. Empirical formulas give the mole ratio of the various elements. However, sometimes different compounds have the same ratio of moles of atoms of the same elements. For example, acetylene, C2H2, and benzene, CeHe, each have 1 1 ratios of moles of carbon atoms to moles of hydrogen atoms. That is, each has an empirical formula CH. Such compounds have the same percent compositions. However, they do not have the same number of atoms in each molecule. The molecular formula is a formula that gives all the information that the empirical formula gives (the mole ratios of the various elements) plus the information of how many atoms are in each molecule. In order to deduce molecular formulas from experimental data, the percent composition and the molar mass are usually determined. The molar mass may be determined experimentally in several ways, one of which will be described in Chap. 12. 

The key to the partitioning of thorium in the described conceptual flow sheet is the solubility in cadmium-magnesium. Experimental data associated with this problem have been obtained. The weight percent thorium in solution was determined by taking samples of the equilibrated liquid at varying temperatures and alloy compositions. The samples were analyzed for thorium, magnesium, and cadmium using atomic absorption spectroscopy and the results are estimated to have a relative accuracy of 3%. 

Percent composition data yield only simplest formulas. To determine the molecular formula for a molecular compound, both its simplest formula and its molecular weight must be known. Some methods for experimental determination of molecular weights are introduced in Chapters 12 and 14. 

If you know the formula of a compound, you can calculate its percent composition. Just the reverse can be done too. If you know the percent composition of a compound, you can calculate a formula for the compound. A formula calculated from percent composition data is called an empirical formula (one calculated from experimental data). The formulas of ionic compounds are always empirical formulas. The formulas of molecular compounds may be the same as their empirical formulas or they may be some whole-number multiple of it. You will learn how to do composition-from-fbrmula and fbrmula-from-composition calculations in this chapter. 

Percent composition from experimental data For example, consider a 100-g sample of a compound that contains 55 g of Element X and 45 g of Element Y. The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100. 

The conversion of experimental composition data to empirical formula is an important step in determining the identity of a substance. Suppose that we have a sample of a heavy, pale yellow, crystalline material that has the composition 59.0% Ba, 13.5% S, and 27.4% O. We need to convert this percent composition data to an empirical formula, which will... 

Rather than giving students straight percent composition data for determining the empirical formula of a compound (see Question 7), sometimes chemistry teachers will try to emphasize the experimental nature of formula determination by converting the percent composition data into actual experimental... 

The percent composition of a compound can be determined (1) from knowing its formula or (2) from experimental data. 

Problem-Solving Strategy for Percent Composition from Experimental Data... 

To determine percent composition from experimental data ... 

Another way to express how much of an element is in a given compound is to use the element s mass percent composition for that compound. The mass percent composition or simply mass percent of an element is the element s percentage of the total mass of the compound. For example, the mass percent composition of sodium in sodium chloride is 39%. This information tells us that a 100-g sample of sodium chloride contains 39 g of sodium. The mass percent composition for a compound can be determined from experimental data using the formula ... 

In the previous section, we learned how to calculate mass percent composition from experimental data and how to use mass percent composition as a conversion factor. We can also calculate the mass percent of any element in a compound from the chemical formula for the compound. Based on the chemical formula, the mass... 

The observed build-up of TATB is similar to that found for Composition B described on page 101. The authors conclude Thus the experimental data obtained on 3 cm long IHE (Insensitive High Explosive) charges may underestimate the C-J pressure by about 20 percent. Attempts to extend the field of equation of state application, for example to model detonations of larger charges, overdriven detonations, or to calculate the sound velocity in the detonation products, will result in appreciable discrepancy between the theory and experiment. ... 

Fig. 12 Cloud-point pressures in the system poly(ethylene-co-l-butene)/propane for different copolymer compositions (B is mole percent butene in the backbone 0% B = LDPE). Polymer weight fraction is about 0.05 wt%. Symbols represent experimental data [54], Lines show PC-SAFT calculations [53]...

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