Mass, SI unit

RMs represent an important tool for demonstrating the traceability of analytical measurements to given references. One should keep in mind that the traceability of chemical analyses is often more difficult to demonstrate than for physical measurements. This is due to major differences in the measurement processes (e.g., matrix influence on chemical analyses, various analytical problems linked to the analytes and the methods used, need for sample pretreatment, etc.). Contrary to physical measurements, the calibrants and CRMs used for chemical analyses are not only used for instrument calibration but also for a variety of other purposes (e.g., method validation). In terms of traceability, the theory implies that the certified values of a substance in a CRM should be traceable to the amoimt of the given substance expressed according to the relevant SI unit, i.e., the mol. Since there is no reference mol , this traceability can be established only in relation to the mass SI unit, i.e., the kg. 

Measurements usually consist of a unit and a number expressing the quantity of that unit. Unfortunately, many different units may be used to express the same physical measurement. For example, the mass of a sample weighing 1.5 g also may be expressed as 0.0033 lb or 0.053 oz. For consistency, and to avoid confusion, scientists use a common set of fundamental units, several of which are listed in Table 2.1. These units are called SI units after the Systeme International d Unites. Other measurements are defined using these fundamental SI units. For example, we measure the quantity of heat produced during a chemical reaction in joules, (J), where... 

There are a few basic numerical and experimental tools with which you must be familiar. Fundamental measurements in analytical chemistry, such as mass and volume, use base SI units, such as the kilogram (kg) and the liter (L). Other units, such as power, are defined in terms of these base units. When reporting measurements, we must be careful to include only those digits that are significant and to maintain the uncertainty implied by these significant figures when transforming measurements into results. 

Two other techniques that depend only on base SI units are coulometry and isotope-dilution mass spectrometry. Coulometry is discussed in Chapter 11. Isotope-dilution mass spectroscopy is beyond the scope of an introductory text, however, the list of suggested readings includes a useful reference. 

In molecular weight determinations it is conventional to dissolve a measured mass of polymer m2 into a volumetric flask and dilute to the mark with an appropriate solvent. We shall use the symbol Cj to designate concentrations in mass per volume units. In practice, 100-ml volumetric flasks are often used, in which case C2 is expressed in grams per 100 ml or grams per deciliter. Even though these are not SI units, they are encountered often enough in the literature to be regarded as conventional solution units in polymer chemistry. 

Mass. The metric ton (symbol t), equal to 1000 kg, is used widely in commerce, although the megagram (Mg) is the appropriate SI unit. 

Moment of Inertia. Moment of inertia, I, is a property of the mass distribution of a body around an axis (I = Smr ). Its SI unit is kg-m. ... 

Centripetal Acceleration. Centripetal acceleration, /r or CO r, where is the tangential linear velocity (m/s), rthe radius (m), and CO the angular velocity (rad/s), is, like any other linear acceleration, measured in SI units m/s. Centripetal force, equal to mass times centripetal acceleration, is, like any force in SI, measured in newtons. 

Dimensionless Quantities. Certain quantities, eg, refractive index and relative density (formerly specific gravity), are expressed by pure numbers. In these cases, the corresponding SI unit is the ratio of the same two SI units, which cancel each other, leaving a dimensionless unit. The SI unit of dimensionless quantities may be expressed as 1. Units for dimensionless quantities such as percent and parts per million (ppm) may also be used with SI in the latter case, it is important to indicate whether the parts per million are by volume or by mass. 

Density and Relative Density. Density is mass per unit volume and in SI is normally expressed as kilograms per cubic meter (density of water = 1000 kg/m or 1 g/cm ). The term specific gravity was formerly the accepted dimensionless value describing the ratio of the density of sohds and Hquids to the density of water at 4°C or for gases to the density of ak at standard conditions. The term specific gravity is being replaced by relative mass density, a more descriptive term. 

Units employed in diffusivity correlations commonly followed the cgs system. Similarly, correlations for mass transfer correlations used the cgs or Enghsh system. In both cases, only the most recent correlations employ SI units. Since most correlations involve other properties and physical parameters, often with mixed units, they are repeated here as originally stated. Common conversion factors are listed in Table 1-4. 

When arriving at the performance of a pump, it is customary to calculate its power output, which is the product of (1) the total dynamic head and (2) the mass of liquid pumped in a given time. In SI units power is expressed in kilowatts horsepower is the conventional unit used in the United States. 

More closely related to health effects and material damage is the energy deposited in a mass of material. The Rad was an early unit the SI unit is the Gray. 

Mole The SI unit of quantity the amount of a pure element or chemical compound that contains the same number of atoms or molecules. It is often simpler to use moles rather than volume or mass when working with gases. Moles are given by... 

It is usual these days to express all physical quantities in the system of units referred to as the Systeme International, SI for short. The International Unions of Pure and Applied Physics, and of Pure and Applied Chemistry both recommend SI units. The units are based on the metre, kilogram, second and the ampere as the fundamental units of length, mass, time and electric current. (There are three other fundamental units in SI, the kelvin, mole and candela which are the units of thermodynamic temperature, amount of substance and luminous intensity, respectively.)... 

Jeffryes, Alec, 628 Joliot, Frederic, 517 Joliot-Curie, Irfcne, 248 Joule The base SI unit of energy, equal to the kinetic energy of a two-kilogram mass moving at a speed of one meter per second, 135,635 Joule, James, 199... 

In this book, we will express our thermodynamic quantities in SI units as much as possible. Thus, length will be expressed in meters (m), mass in kilograms (kg), time in seconds (s), temperature in Kelvins (K), electric current in amperes (A), amount in moles (mol), and luminous intensity in candella (cd). Related units are cubic meters (m3) for volume, Pascals (Pa) for pressure. Joules (J) for energy, and Newtons (N) for force. The gas constant R in SI units has the value of 8.314510 J K l - mol-1, and this is the value we will use almost exclusively in our calculations. 

A note on good practice We use SI units throughout R is in its fundamental SI form and the molar mass is in SI base units to be consistent with the choice for R. The cancellation of the units has made use of the relation 1 J = 1 kg-ur-s 2. 

Kelvin scale A fundamental scale of temperature on which the triple point of water lies at 273.16 K and the lowest attainable temperature is at 0. The unit on the Kelvin scale is the kelvin, K. ketone An organic compound containing a carbonyl group between two carbon atoms, having the form R—CO R. Example CH3—CO—CH2CH , butanone. kilogram (kg) The SI unit of mass. See also Appendix IB. 

Here, kg and ki are the gas-side and liquid-side mass transfer coefficients. Their units are identical to those for Kg and Ki, m/s. Like the overall coefficients, they are usually measured and reported as the composite quantities kgAj and kiAi with SI units of s. ... 

Here, / is the electric field, k is the electrical conductivity or electrolytic conductivity in the Systeme International (SI) unit, X the thermal conductivity, and D the diffusion coefficient. is the electric current per unit area, J, is the heat flow per unit area per unit time, and Ji is the flow of component i in units of mass, or mole, per unit area per unit time. 

All equations given in this text appear in a very compact form, without any fundamental physical constants. We achieve this by employing the so-called system of atomic units, which is particularly adapted for working with atoms and molecules. In this system, physical quantities are expressed as multiples of fundamental constants and, if necessary, as combinations of such constants. The mass of an electron, me, the modulus of its charge, lei, Planck s constant h divided by lit, h, and 4jt 0, the permittivity of the vacuum, are all set to unity. Mass, charge, action etc. are then expressed as multiples of these constants, which can therefore be dropped from all equations. The definitions of atomic units used in this book and their relations to the corresponding SI units are summarized in Table 1-1. 

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