Amount of substance - the mole

The amount of substance, the mole (formerly referred to as the gram-mole), symbol mol, is that amount of a substance of a system which contains as many elementary entities as there are in 0.012 kilogram of carbon-12. The elementary entities—atoms, molecules, ions, etc. —must be identified when the mole is used. Thus, the conventional gram-mole of carbon-12 contains Avogadro s number of atoms, which amounts to 0.012 kg or 12g, obtained by multiplying as follows ... 

The unit that underpins chemical measurements is the unit of amount of substance, the mole. However, in practice, as there is no mole standard, the kg is used, i.e. chemical measurements are actually traceable to the mass unit, the kg. In other words, water-related chemical measurements are based on the determination of amount of substance per mass of matrix. For solid matrices (sediment, suspended matters, biota), these are units corresponding to ultratrace (ng/kg) and trace (ftg/kg) concentrations for many organic micropollutants and trace elements, and mg/kg for major elements. For water, results should also in principle be reported in mass/kg of water but the practice is usually that they are reported as mass/volume, e.g. ng/1, pg/l or mg/1, which is already diverging from basic metrological principles. 

CRMs, at least those which can claim that they are traceable to a recognised reference or standard, e.g. the S.I. unit, can be used to link the measurement to this reference. Such CRMs can be compared to transfer standards, as they are known and used in physics, e.g. mass transfer standards. Unfortunately, except for pure primary substances, such materials hardly exist in chemical measurements for complex materials. Pure materials are often the only real link to the basic S.I. unit of amount of substance, the mole, for many measurement processes. They intervene in fact mainly in the calibration process (see Figure 2.17). Finally, some CRMs, e.g. those used to test material properties or activities (e.g. pH, conductivity, etc.) can be used to realise measurement scales for these properties. The last two structural roles in measurement sciences also represent the primary practical role of CRMs as they intervene directly in the measurement process. 

Amount of substance The mole (symbol mol) is the SI unit for the amount of a substance and is equal to the number of atoms in 0.012 kg of carbon-12 atoms. 

Mol is the abbreviation for mole, the SI unit for amount of substance. The mole is introduced in Chapter 7. 

If we denote the number of objects by N and the amount of substance (in moles) by n, this relation is written... 

Sections address how chemists answer the question, What is it It is just as important to ask, How much is there As described in Section 2-, chemists use the mole to describe amounts of substances. The procedures introduced in that section work equally well for compounds as for elements. To apply these procedures, we need to be able to determine the molar masses of chemical compounds. 

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). 

SI units of measurement, used by scientists around the world, derive their name from the French Systeme International d Unites. Fundamental units (base units) from which all others are derived are defined in Table 1-1. Standards of length, mass, and time are the meter (m). kilogram (kg), and second (s), respectively. Temperature is measured in kelvins (K), amount of substance in moles (mol), and electric current in amperes (A). 

MOLE (mol). A unit of amount of substance. One mole is an amount of a substance, in specified mass units, equal to the molecular weight of that substance. (The SI unit for amount of substance. Examples are the gram mole or the pound mole.)... 

Any chemical experiment involves the reaction of enormous numbers of atoms or molecules. The term mole is used to indicate a collection of a large, fixed number of fundamental chemical entities, comparable to the quantity that might be involved in an actual experiment. In fact, the mole is recognized in SI as the unit for one of the dimensionally independent quantities, the amount of substance. The abbreviation for the unit is mol. A mole of atoms of any element is defined as that amount of substance containing the same number of atoms as there are carbon atoms in exactly 12g of pure 12C. This number is called Avogadro s number or Avogadro s constant, Na. The value of this quantity may be related to the value of the u, listed in Table 2-1, as follows ... 

Mole A mole is a measure of amount of substance. One mole is the formula weight of the substance expressed in grams. For example, for limonene, formula C10H16, the formula weight is (C = 12) (10 x 12) + (16 x 1) (H = 1) = 136 so that one mole of limonene is 136 grams of the compound. One mole of any substance contains the same number of units (atoms, molecules or ions). This is termed the Avogadro number, 6.022 x 1023 in scientific notation. 

Consider an analytical method involving the titration of hydrochloric acid with anhydrous sodium carbonate to determine the concentration of the acid. The measurements made are mass (weighing out a chemical to make up a solution of known concentration) and volume (dispensing liquids with pipettes and burettes). The reaction between the two chemicals is based on amount of substance - one mole of sodium carbonate reacts with two moles of hydrochloric acid - and the mass of a mole is known (e.g. the formula weight in grams of one mole of sodium carbonate is 105.99). All the measurements are based on either length or mass and are traceable to SI units, so the method is a primary method. 

Chemists have adopted a special unit of measurement called the mole (abbreviation mol) to describe the amount of substance. A mole is defined as 6.022 136 7 x 102i molecules or atoms. This is chosen to be equivalent to its molecular weight in grams. Thus 1 mol of sodium, which has an atomic weight of 23 (if one were to be very accurate, it would be 22.9898), weighs 23 g and contains 6.022 136 7 x 1023 molecules. This special number of particles is called the Avogadro number, in honour of the Italian physicist Amedeo Avogadro. 

In a similar way, we use Avogadro s number, which expresses the equivalent relationship between 1 mole of a substance and the number of entities it contains, as a conversion factor. We multiply by Avogadro s number to convert amount of substance (in moles) to the number of entities (atoms, molecules, or formula units) ... 

Since any tangible sample of matter contains such an enormous number of atoms or molecules, a unit larger than the entity itself is needed to measure the amount of substance. The SI unit for amount of substance is the mole. The mole is defined as the amount of substance in exactly 0.012 kg of carbon-12. One mole of any substance contains the same number of elementary entities as there are carbon atoms in exactly 0.012 kg of carbon-12. This number is the Avogadro constant, = 6.022045 x 10 mol . 

To count extremely small particles, such as atoms, Avogadros number must be an enormous quantity. As you might imagine, Avogadros number would not be convenient for measuring a quantity of marbles. Avogadros number of marbles would cover the surface of Earth to a depth of more than six kilometers Figure 10.2, however, shows that it is convenient to use the mole to measure amounts of substances. One-mole quantities of water, copper, and salt are shown, each with a different representative particle. The representative particle in a mole of water is the water molecule, the representative particle in a mole of copper is the copper atom, and the representative particle in a mole of sodium chloride is the NaCl formula unit. 

Furio, C., Azcona, R. Ratcliffe, M. (2000). Difficulties in teaching the concepts of amount of substance and mole . International Journal of Science Education, 22, 1285-1304. 

Since the amount of substance in moles = volume X molar concentration, we can substitute ... 

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