Using and Converting Units

Wave you ever been asked for your height in centimeters, your weight in kilograms, or the speed limit in kilometers per hour These measurements may seem a bit odd to those folks who are used to feet, pounds, and miles per hour, but the truth is that scientists sneer at feet, pounds, and miles. Because scientists around the globe constantly communicate numbers to each other, they prefer a highly systematic, standardized system. The International System of Units, abbreviated SI from the French term Systeme International, is the unit system of choice in the scientific community. 

In this chapter, you find that the SI system offers a very logical and well-organized set of units. Scientists, despite what many of their hairstyles may imply, love logic and order, so SI is their system of choice. 

As you work with SI units, try to develop a good sense for how big or small the various units are. That way, as you re doing problems, you ll have a sense for whether your answer is reasonable. 

Familiarizing loursetf ulith Base Units and Metric System Prefixes 

The first step in mastering the SI system is to figure out the base units. Much like the atom, the SI base units are building blocks for more-complicated units. In later sections of this chapter, you find out how more-complicated units are built from the SI base units. The five SI base units that you need to do chemistry problems (as well as their familiar, non-SI counterparts) are in Table 2-1. 

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SI (le Systeme International d UniUs) units are used in many countries to express clinical laboratory and serum drug concentration data. Instead of employing units of mass (such as micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mol (the molecular weight of the substance in grams) of the solute in 1 L of solution. The following formula is used to convert units of mass to moles (mcg/mL to pmol/L or, by substitution of terms, mg/mL to mmol/L or ng/mL to nmol/L). 

Appendices A, B, and C are important pedagogically. Appendix A discusses experimental error and scientific notation. Appendix B introduces the SI system of units used throughout the book and describes the methods used for converting units. Appendix B also provides a brief review of some fundamental principles in physics,... 

The excited-state reduction potential, °( Cr3+/Cr2+), can be estimated using an analysis similar to Hess s law of heat summation (Fig. 8.5). Using the emission maximum (730 nm) in the luminescence spectrum and converting units yields an excited-state energy of 164 kJ mol-1 for [Cr(phen)3]3+. That means that relaxation of the 2E excited state to the ground state involves AG° = 164 kJ mol-1 or a one-electron electro-... 

Mathcad (http //www.mathcad.com) provides hundreds of operators and built-in functions for solving technical problems. Mathcad can be used to perform numeric calculations or to find symbolic solutions. It automatically tracks and converts units and operates on scalars, vectors, and matrices. 

Lighter C —Cg a-olefias and Cg branched olefins are converted by the oxo process into fatty acids containing one carbon number greater than the starting a-olefin. These fatty acids are then used to produce alkenylhen enesulfonic acid products which are used ia the United States and ia Europe as perborate bleach activators ia heavy-duty laundry detergents. 

Figure 2 shows a general process flow diagram for almost all production of natural sodium sulfate. Glauber s salt can be converted to anhydrous sodium sulfate by simply drying it in rotary kilns. Direct drying forms a fine, undesirable powder, and any impurities in the Glauber s salt become part of the final product. This process is not used in the United States but is used in other countries. 

A chlorination process (20,21,44—46) converts sucrose into sucralose [56038-13-2] (4,l, 6 -trichloro-4,l, 6 -trideoxy-galactosucrose), a heat-stable, noncariogenic, noncaloric, high intensity sweetener. Sucralose is approved for food use in Canada, Australia, and Russia. It is not yet approved for use in the United States. 

Other burners are used for low capacity operations. A cascade or checker burner, ia which molten sulfur flows down through brick checkerwork countercurrent to a flow of air, is used ia small units with a sulfur trioxide converter to condition gases entering electrostatic precipitators at boiler plants operating on low sulfur coal. A small pan burner, which is fed with soHd, low carbon sulfur, is used to produce sulfur dioxide for solution ia irrigation water to control the pH and maintain porosity ia the soil. The same type of burner is used to disiafect wastewater ia this case sulfur dioxide is used iastead of chlorine. 

The Claus process is the most widely used to convert hydrogen sulfide to sulfur. The process, developed by C. F. Claus in 1883, was significantly modified in the late 1930s by I. G. Farbenindustrie AG, but did not become widely used until the 1950s. Figure 5 illustrates the basic process scheme. A Claus sulfur recovery unit consists of a combustion furnace, waste heat boiler, sulfur condenser, and a series of catalytic stages each of which employs reheat, catalyst bed, and sulfur condenser. Typically, two or three catalytic stages are employed. 

The Henkel process provides a means to convert toluene to benzene and at the same time makes use of the methyl group. Neither of these two processes is economically attractive for use in the United States. 

Recycling. In more recent years, processes that can convert used carbide cutting tools and used tungsten alloy penetrators back into powdered form that can be used directly into new products have been developed. It is estimated that in 1996 ca 25% of cutting inserts used in the United States were recycled in this way. 

Unear Umts. The following procedure is used for converting linear units to the proper number of significant places the maximum and minimum limits in inches are calculated. The corresponding two values are converted exacdy into millimeters by multiplying each by the conversion factor 1 in. = 25.4 mm. The results are rounded in accordance with Table 4. 

In the BASF synthesis, a Wittig reaction between two moles of phosphonium salt (vitamin A intermediate (24)) and C q dialdehyde (48) is the important synthetic step (9,28,29). Thermal isomerization affords all /ra/ j -P-carotene (Fig. 11). In an alternative preparation by Roche, vitamin A process streams can be used and in this scheme, retinol is carefully oxidized to retinal, and a second portion is converted to the C2Q phosphonium salt (49). These two halves are united using standard Wittig chemistry (8) (Fig. 12). 

The mother Hquoi obtained from the crystallization, or the raffinate after removal by adsorption, is isomerized using an acidic catalyst to convert xylene to the o- and -isomers (Unit K in Fig. 8). 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

LiquidPha.se. The methyl chloride process with the widest use in the United States is the Hquid-phase methanol hydrochlorination process. SHicone producers use methyl chloride in its manufacture and produce an aqueous hydrochloric acid stream as a by-product. This by-product HCl is converted back to methyl chloride by hydrochlorination. In fact, it is possible to produce methyl chloride directiy from the chioromethylsilane hydrolysis step in the siHcone process (18,19) (see Silicon compounds, silicones). 

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