Amount of a substance

Freundlich isotherm The empirical relationship between the amount of a substance adsorbed and the concentration of the solute... 

A second use of arrays arises in the detection of trace components of material introduced into a mass spectrometer. For such very small quantities, it may well be that, by the time a scan has been carried out by a mass spectrometer with a point ion collector, the tiny amount of substance may have disappeared before the scan has been completed. An array collector overcomes this problem. Often, the problem of detecting trace amounts of a substance using a point ion collector is overcome by measuring not the whole mass spectrum but only one characteristic m/z value (single ion monitoring or single ion detection). However, unlike array detection, this single-ion detection method does not provide the whole spectrum, and an identification based on only one m/z value may well be open to misinterpretation and error. 

Array detectors are particularly useful for detecting ions from either a very small amount of a substance or when ionization is not continuous but intermittent. 

For carckiogen pesticides (70,71), animal testkigs are subject to maximum tolerated doses (M I L)). M I D is the maximum amount of a substance that can be administered to an experimental animal without causkig extreme health consequences, such as death, to occur but while continuing to produce some measurable toxic effects. Current regulatory theory holds that carckiogen effects do not have a threshold and caimot be related to reference doses. 

Although the EEC is stiU in the process of completing cosmetic regulation, the final directive is expected to require member states to ban marketing of cosmetics that contain prohibited ingredients, an amount of a substance in excess of that proscribed, coloring agents, preservatives, or uv filters not specifically allowed. 

Retains The amount of a substance left behind during chemical processing. Also refers to samples retained for reference after undergoing analysis. 

Concentration The amount of a substance present in a given volume of a gas or liquid, in parts per million (ppm) or jLg m . In the case of gases, the ppm is proportional to the molecular concentration, hence the relationship between ppm and pg m " depends on the molecular weight of the gas concerned. 

The equivalent weight of an ion (or an element) is the ratio of its formula weight to its valence. According to an alternative definition that is also suitable for compounds, an equivalent weight represents the amount of a substance which will react with one atomic weight of hydrogen or its chemical equivalent. 

The method can also be adapted to determine the amount of a substance in solution by adding a catalyst which will destroy it completely, and measuring the concomitant change in for example, the absorbance of the solution for visible or ultraviolet radiation. Such procedures are applied in clinical chemistry. 

Precipitation reactions have many applications. One is to make compounds. The strategy is to choose starting solutions that form a precipitate of the desired insoluble compound when they are mixed. Then we can separate the insoluble compound from the reaction mixture by filtration. Another application is in chemical analysis. In qualitative analysis—the determination of the substances present in a sample—the formation of a precipitate is used to confirm the identity of certain ions. In quantitative analysis, the aim is to determine the amount of each substance or element present. In particular, in gravimetric analysis, the amount of substance present is determined by measurements of mass. In this application, an insoluble compound is precipitated, the precipitate is filtered off and weighed, and from its mass the amount of a substance in one of the original solutions is calculated (Fig. 1.6). Gravimetric analysis can be used in environmental monitoring to find out how much of a heavy metal ion, such as lead or mercury, is in a sample of water. 

This relation can be used to estimate the molar mass of a substance by comparing the time required for a given amount of the unknown substance to effuse with that required for the same amount of a substance with a known molar mass. 

Exponential decay is quite regular starting with a given amount of a substance at t = 0, this amount will fall to V2 its original value after one half-life, to 1/4 after two half-lives, Vs after three half-lives, and so forth. This regularity has its usefulness, and the decay of has been widely employed to date archeological artifacts [3]. 

Unlike mass and volume, density does not vary with the amount of a substance. Notice in Figure 1-20 that all the corks float, regardless of their sizes. Notice also that all the pieces of lead sink, regardless of their sizes. Dividing a sample into portions changes the mass and volume of each portion but leaves the density unchanged. A property that depends on amount is called extensive. Mass and volume are two examples of extensive properties. A property that is independent of amount is called intensive. Density and temperature are intensive properties. 

Despite Lavoisier s early work on the link between energy and life, calorimetric measurements played a relatively minor role in biology until recent years, primarily because of practical obstacles. Every organism must take in and give off matter as part of its normal function, and it is very difficult to make accurate heat-flow measurements when matter is transferred. Moreover, the sizes of many organisms are poorly matched to the sizes of calorimeters. Although a chemist can adjust the amount of a substance on which to carry out calorimetry, a biologist often cannot. 

ADI = acceptable daily intake, estimate of amount of a substance in food or drinking water, expressed as mg/kg body weight, that can be ingested daily over a lifetime without appreciable risk (weight of standard human = 60 kg) bw = body weight. 

Influencing the efficacy or potency of chemicals is a strategy used by the pharmaceutical industry as part of the drug discovery process that can be incorporated into designing safer industrial chemicals. Efficacy is the maximal effect, either therapeutic or toxic, that a chemical can achieve. Potency is a measure of the amount of a substance that is needed to attain a given response level. Opioid analgesics are examples of where structural modifications have been used to establish a relationship between structure and activity. ... 

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). 

Absorbed Dose, Chemical—The amount of a substance that is either absorbed into the body or placed in contact with the skin. For oral or inhalation routes, this is normally the product of the intake quantity and the uptake fraction divided by the body weight and, if appropriate, the time, expressed as mg/kg for a single intake or mg/kg/day for multiple intakes. For dermal exposure, this is the amount of material applied to the skin, and is normally divided by the body mass and expressed as mg/kg. 

Biological Half-time—The time required for a biological system, such as that of a human, to eliminate by natural process half of the amount of a substance (such as a chemical substance, either stable or radioactive) that has entered it. 

The unit mol/L stands for moles per liter. A mole is a measurement that chemists use to state the amount of a substance. One mole is equal to 6.02 x 1023 of anything. For example, 6.02 x 1023 atoms of carbon are equal to 1 mole (mol) of carbon. One mole of hydrogen ions would equal 6.02 x 1023 hydrogen ions. And 6.02 x 1023 sandwiches would equal 1 mole of sandwiches. Using hydrogen ion and hydroxide ion concentrations in moles per liter can be cumbersome, however. Instead, chemists use the pH of a solution to describe the hydrogen and hydroxide ion concentrations. 

The tube contains the thermodynamic quantity in an amount M (amount of a substance, thermal energy, etc.), which has a density (concentration, energy density, etc.) p(x) at each point in the tube defined by the relationship... 

The simplest and most widely used method for predicting adsorption is to measure adsorption isotherms (the variations in the amount of a substance adsorbed at different concentrations measured at a constant temperature). Empirical constants can be calculated from such measurements. 

When determining the impact of a stressor using this approach, there are two main steps, exposure assessment and hazard assessment [28]. The exposure assessment is aimed to determine the concentrations of a substance that a certain organism can be exposed to due to the emission of a certain amount of a substance. The first step is the release estimation. The release estimation aims to determine how much of... 

An equivalent is defined as that amount of a substance that reacts with or produces a mole of what ... 

The fundamental unit in chemical measurement is the mole - amount of substance. A mole is the amount of a substance that contains as many atoms, molecules, ions or other elementary units as the number of atoms in 0.012 kg of carbon 12 (12C). It is the only dimensionless SI unit. In practical terms, it is almost impossible to isolate a mole of pure substance. Substances with a purity of better than 99.9% are rare one exception is silver, which can be obtained with a purity of 99.9995% which is referred to as five nines silver . Another problem is that it is not always possible to isolate all of the analyte from the sample matrix, and the performance of the chemical measurement may be matrix-dependent - a given response to a certain amount of a chemical in isolation may be different from the response to the same amount of the chemical when other chemicals are present. If it is possible to isolate quantitatively all of the analyte of interest from the accompanying sample matrix, then a pure chemical substance may be used for calibration. The extent to which the analyte can be recovered from the sample matrix will have been determined as part of the method validation process (see Chapter 4, Section 4.6.3). 

All equipment has limitations, for example, the amount of a substance it can detect or the accuracy with which it can make a measurement. If you attempt to make the equipment perform beyond its capabilities, it does not matter how carefully the equipment is operated, it will not be possible to get meaningful results. In terms of a particular instrument, fitness for purpose is interpreted as having appropriate performance capability to do the work required. This applies to all equipment, large or small. For example, a stirrer needs to perform the intended task satisfactorily while remaining essentially inert. There is a formal process for assessing the suitability of equipment to perform a given task - this is called Equipment Qualification or Equipment Validation. This is dealt with in Section 5.6.3. 

A mole is the amount of a substance that contains as many elementary particles as there are atoms in 12 g of the carbon-12 isotope. 

To a chemist, the amount of a substance is measured in moles (abbreviated to mol). Relative atomic masses (or molar masses in gmol ) do not have to be learnt because examination groups always include them on question papers. 

The amount of a substance present in a given mass or volume of another substance. The abbreviations w/w, w/v and v/v are sometimes used to indicate whether the concentration quoted is based on the weights or volumes of the two substances. Concentration may be expressed in several ways. These are shown in Table 1.2. 

That amount of a substance which, in a specified chemical reaction, produces, reacts with or can be indirectly equated with one mole (6.023 x io23) of hydrogen ions. This confusing term is obsolete but its use is still to be found in some analytical laboratories. 

The amount of a substances present in the plant (i.e. inventory) has a large effect on the degree of hazard. It is advised to use a minimum storage inventory of... 

This simple calculation illustrates the fundamental truth underlying neutralization reactions complete reaction requires equal amounts of acid and alkali. In fact, the primary purpose of a titration is to measure an unknown amount of a substance in a sample, as determined via a chemical reaction with a known amount of a suitable reagent. We perform the titration to ascertain when an equivalent amount of the reagent has been added to the sample. When the amount of acid and alkali are just equal, we have the equivalence point, from which we can determine the unknown amount. 

The mole (mol) is the amount of a substance that contains the same number of particles as atoms in exactly 12 grams of carbon-12. This number of particles (atoms, molecules, or ions) per mole is Avogadro s number and is numerically equal to 6.022 x 1023 particles. The mole is simply a term that represents a certain number of particles, like a dozen or a pair. The mole also represents a certain mass of a chemical substance. 

Since the peak size is directly proportional to concentration, one may think that one could prepare a series of standard solutions and obtain peak sizes to be used for a standard curve of peak size vs. concentration, a method similar to Beer s law in spectrophotometry, for example. But since peak size also varies with amount injected, there can be considerable error due to the difficulty in injecting consistent volumes, as discussed above and in Section 12.3. A method that does away with this problem is the internal standard method. In this method, all standards and samples are spiked with a constant known amount of a substance to act as what is called an internal standard. The purpose of the internal standard is to serve as a reference for the peak size measurements, so that slight variations in injection... 

The mole (mol) is the amount of a substance that contains the same number of particles as atoms in exactly 12 grams of carbon-12. This number of particles (atoms or molecules or ions) per mole is called Avogadro s number and is numerically equal to 6.022 x 1023 particles. The mole is simply a term that represents a certain number of particles, like a dozen or a pair. That relates moles to the microscopic world, but what about the macroscopic world The mole also represents a certain mass of a chemical substance. That mass is the substance s atomic or molecular mass expressed in grams. In Chapter 5, the Basics chapter, we described the atomic mass of an element in terms of atomic mass units (amu). This was the mass associated with an individual atom. Then we described how one could calculate the mass of a compound by simply adding together the masses, in amu, of the individual elements in the compound. This is still the case, but at the macroscopic level the unit of grams is used to represent the quantity of a mole. Thus, the following relationships apply ... 

The amount of a substance present in a sample is determined by taking a solution containing that substance and precipitating a compound containing that substance. The precipitate is then dried and weighed. (See the Stoichiometry chapter.)... 

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