The Base Units

This book focuses in on the original base units (length, mass, and temperature), plus one of the derived units (volume), because these measurements are the most commonly used measurements in the lab. Time is included in this section only because it provides an interesting commentary on the metrologists desire to split hairs in their endeavor to achieve accuracy. 

Length. The original metric standard for the length of a meter was one ten-millionth part of a quadrant of the earth s meridian. Astronomical measurements (at that time) indicated that one-tenth of a quadrant of the earth s meridian was measured between Dunkirk, in France, and a point near Barcelona, Spain. This distance was dutifully measured and divided by one million to obtain the meter. It is fortunate that the meter was later redefined, because if you take what we now call a meter and measure the distance between the Dunkirk and Barcelona points used, you get the length space of 1,075,039 meters. This space is an accuracy of only 7.5 in 100 meters. 

Reproducible accuracy was increased substantially by the development and use of the International Prototype Metre, the platinum-iridium bar. By physically 

In 1983 yet another definition of the meter was adopted It was the distance that light would travel (in a vacuum) during 1/299,792,458 of a second. This attempt provided a measurement ten times more accurate than that obtained through kryp-ton-86 techniques. This meter, accurate to within four parts in one billion, is still in use today. 

It has been hoped that there will be some naturally occurring phenomenon in nature to which we can ascribe the value of the kilogram, thus allowing the kilogram to be based on a reproducible phenomenon rather than relying on a physical 

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In order to increase the precision of realization of the base unit meter, the definition based on the wavelength of a krypton-86 radiation was replaced in 1983 by one based on the speed of light. Also added were the prefixes zetta (Z) for 10, zepto (z) for 10 , yotta (Y) for 10 , and yocto (y) for 10 . 

Compound Units. It is usually recommended that only one prefix be used in forming a multiple of a compound unit, and that it should be attached to the numerator. An exception is the base unit kilogram, where it appears in the denominator. Multiples of kilogram are formed by attaching the prefix to the word gram (g). Compound prefixes are not used eg, 1 pF is correct, not 1 )J.)J.F. 

The word concentration is frequently used as a general term referring to a quantity of substance in a defined volume of solution. But for quantitative titrimetric analysis use is made of standard solutions in which the base unit of quantity employed is the mole. This follows the definition given by the International Union of Pure and Applied Chemistry1 in which ... 

Units may be combined together into derived units to express a property more complicated than mass, length, or time. For example, volume, V, the amount of space occupied by a substance, is the product of three lengths therefore, the derived unit of volume is (meter)3, denoted m3. Similarly, density, the mass of a sample divided by its volume, is expressed in terms of the base unit for mass divided by the derived unit for volume—namely, kilogram/(meter)3, denoted kg/m3 or, equivalently, kg-m-3. The SI convention is that a power, such as the 3 in cm3, refers to the unit and its multiple. That is, cm3 should be interpreted as (cm)3 or 10-6 m3 not as c(m3), or 10 2 m3. Many of the more common derived units have names and abbreviations of their own. 

Derived units are combinations of the base units (Sec- with a lowercase letter but their abbreviations are upper-tion A). Table 4 lists some derived units. Note that the case, names of units derived from the names of people all begin... 

The international scientific community prefers to work exclusively with a single set of units, the Systeme International (SI), which expresses each fundamental physical quantity in decimally (power of 10) related units. The seven base units of the SI are listed in Table 1-3. The SI unit for volume is obtained from the base unit for length A cube that measures 1 meter on a side has a volume of 1 cubic meter. 

Inductors—be aware that many vendors put cryptic markings such as 102 or 103 on them. For capacitors, there are industry standard markings. For example, 221 is 22 x 1 O pf, 222 is 22 x 102pf, and so on. All are referred to the base unit, pF. But in inductors, 102 may be 10 x 102in nH or pH. In other words they could be a factor of 1000 apart, with the same marking. If necessary, find an LCR meter and double-check. 

Figure 1.39 The formation of an N-glycosidic bond links the base unit of nucleic acids to the associated ribose derivative.

The a-carbons of nitrosamines undergo enzymatic hydroxylation followed by oxidative cleavage leading to the formation of alkyldiazo hydroxides, alkyldiazonium ions and alkyl cations [34]. These cations are postulated to initiate the process of carcinogenesis in some cases by alkylating the base units of DNA [35, 36]. 

The measurement system that you will most likely encounter is the SI (Metric) system. Each quantity (such as mass and volume) has a base unit and a prefix that modifies the base unit. The prefixes are the same for all quantities and are based on a decimal system. Below are some basic SI units we will introduce others in later chapters ... 

The measurement system that is most widely used in chemistry is the SI system. It incorporates a base unit for the various quantities and then uses prefixes to moderate the value of the base unit. The Unit Conversion Method is a way to generate easily the setup to a problem. Be sure to round off the final answer to the correct number of significant figures and include the units with the final answer. 

Viscosity, as it was defined in Equation (15.1), is often called the dynamic viscosity. The most common unit of dynamic viscosity is the centipoise, a unit based on force per rate. The base unit, the poise (100 centipoise per poise) is seldom used. Of more practical significance is kinematic viscosity, which is the dynamic viscosity divided by the density of the fluid ... 

The most common unit of kinematic viscosity is the centistokes (cS), the centipoise per density unit. Again, the base unit, the stokes (S) (100 cS/S), is seldom used. 

The system of units used in chemistry is the SI system (Systeme International), which is related to the metric system. There are base units for length, mass, etc. and decimal prefixes that modify the base unit. Since most of us do not tend to think in these units, it is important to be able to convert back and forth from the English system to the SI system. These three conversions are useful ones, although knowing the others might allow you to simplify your calculations ... 

The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of the International System of Units (SI) in all scientific and technical publications [13]. Appendix A list the names and symbols adopted for the seven SI base units, together with several SI derived units, which have special names and are relevant in molecular energetics. Among the base units, the kelvin (symbol K) and the mole (mol), representing thermodynamic temperature and amount of substance, respectively, are of particular importance. Derived units include the SI unit of energy, the joule (J), and the SI unit of pressure, the pascal (Pa). 

We are normally used to x being represented in equations as the base unit (i.e. y = x2). In the exponential function, it becomes the exponent (y=ex), which conveys some very particular properties. 

A conversion factor of 80 is used to convert from the base units in the equation (mmHg and l.min-1) to the commonly used units of the result (dyne.s.cm-5). It is the pressure difference between input (CVP or RAP) and output (MAP) that is used in these equations rather than simply SBP. The SVR is usually 1000-1500 dyne.s.cm-5. 

The base units of measurement under the Systeme International d Unites, or SI units, are given in Table 2.1 [3]. 

The International Union for Pure and Applied Chemistry (lUPAC) recommends the adoption in scientific publications of the SI system notation. Tables A 1.1 and A1.2 present the base units and some derived units. 

The mole was adopted as the seventh SI base unit in 1971. An important factor of the SI system of units is coherence, by which is meant that derived units are defined by the multiplication and/or division of the base units, without the need for any numerical factors. 

Secondary structure of DNA consists of two strands of polynucleotides coiled around each there in the form of double helix. The backbone of each strand is sugar-phosphate unit and the base unit of each strand are pointed into the interior of the helix and are linked through H-bonds. G and C are held by three H-bonds, A and T are held by two bonds. Unlike DNA, RNA has a single strand. 

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. 

Chemists routinely measure quantities that run the gamut from very small (the size of an atom, for example) to extremely large (such as the number of particles in one mole). Nobody, not even chemists, likes dealing with scientific notation (which we cover in Chapter 1) if they don t have to. For these reasons, chemists often use a metric system prefix (a word part that goes in front of the base unit to indicate a numerical value) in lieu of scientific notation. For example, the size of the nucleus of an atom is roughly 1 nanometer across, which is a nicer way of saying 1x10- meters across. The most useful of these prefixes are in Table 2-2. 

This conversion requires you to move across the metric-system prefixes you find in Table 2-2. When you re working on a conversion that passes through a base unit, it may be helpful to treat the process as two steps, converting to and from the base unit. In this case, you can convert from millimeters to meters and then from meters to kilometers ... 

The most common prefixes used in medicine are kilo, which is 1,000 times the base unit, centi, which is 1/100 of the base unit, and milli, which is 1/1,000 times the base unit. These prefixes are attached to the base unit to give the multiple of the base unit being measured. Therefore, a kilogram is 1,000 grams. A centimeter is 1/100 of a meter. A milliliter is 1/1,000 of a liter. 

In the alternative atomic units (a.u.) favored by electronic structure theorists, the base units (me = e = h= 1) are instead identified with fundamental physical objects of the... 

Place the electrode holder with the IEF electrodes on the electrophoresis apparatus and align electrodes over the center of the electrode strips. Connect the two electrodes to the base unit and place the lid on the apparatus. 

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