Usage and Importance of Amount of Substance
– The mole was historically defined as the amount of substance in 12 grams of the carbon-12 isotope.
– Working with amounts of substances (moles or molecules) is often more convenient than working with masses or volumes.
– Concentrations of reagents and products in solution are often specified in moles per liter.
– The amount of substance is a convenient concept in thermodynamics, as it is directly related to the number of molecules in a gas.
Nature of Particles and Mass
– The nature of the particles should be specified when measuring the amount of substance.
– A sample of 1 mole of molecules of oxygen has a mass of about 32 grams.
– A sample of 1 mole of atoms of oxygen has a mass of about 16 grams.
Derived Quantities
– The quotient of an extensive physical quantity by the amount of substance is an intensive property, usually named with the prefix ‘molar’ or the suffix ‘per mole’.
– Examples include molar mass, molar volume, and molar heat capacity.
– Molar mass is the ratio of the mass of a sample to its amount of substance.
– Molar concentration is the number of moles per unit of volume in a solution.
– Molar fraction is the number of moles of a substance in a mixture divided by the total number of moles of all components.
Molar Quantities (per mole)
– Molar mass is the ratio of the mass of a sample to its amount of substance.
– Molar volume is the ratio of the volume of a sample to its amount of substance.
– Molar heat capacity is the ratio of the heat capacity of a sample to its amount of substance.
– Molar mass can be determined by measuring mass, molar volume, or electric charge.
– The number of moles in a sample is obtained by dividing the mass of the sample by the molar mass of the compound.
Amount Concentration (moles per liter)
– Amount concentration is the amount of a specific substance in a sample of a solution divided by the volume of the sample.
– The SI unit for amount concentration is moles per liter.
– Amount concentration is commonly referred to as molarity, denoted by the symbol M.
– Amount concentration should not be confused with mass concentration.
– Molarity is not condoned by IUPAC, but it is commonly used in chemistry.
History and Development of the Concept of Amount of Substance
– Alchemists and early metallurgists had some notion of amount of substance, but no generalization beyond recipes.
– In 1758, Mikhail Lomonosov questioned the idea that mass was the only measure of the quantity of matter.
– The concept of amount of substance was developed alongside the birth of modern chemistry.
– In 1777, Wenzel published ‘Lessons on Affinity,’ demonstrating the proportions of base and acid components in reactions.
– Lavoisier published ‘Treatise of Elementary Chemistry’ in 1789, introducing the concept of a chemical element and the Law of conservation of mass.
– In 1792, Richter published the first volume of ‘Stoichiometry or the Art of Measuring the Chemical Elements,’ using the term stoichiometry for the first time.
– Proust’s Law of definite proportions in 1794 generalized the concept of equivalent weights.
– Dalton published his first paper on modern atomic theory in 1805, including a table of relative weights of particles.
– In 1808, Dalton published ‘A New System of Chemical Philosophy,’ containing the first table of atomic weights.
– Gay-Lussac’s Law of combining volumes in 1809 established the relationship between volumes of reactants and products in gas reactions.
– In 1811, Avogadro hypothesized equal volumes of different gases contain equal numbers of particles.
– Berzelius published the first tables of atomic weights based on the scale of (O) = 100 in 1813/1814.
– Prout’s hypothesis in 1815 suggested all atomic weights are integer multiples of the atomic weight of hydrogen.
– Dulong-Petit law in 1819 related atomic weight of a solid element to its specific heat capacity.
– Mitscherlich’s work on crystal isomorphism in 1819 clarified chemical formulae and atomic weights.
– In 1834, Clapeyron stated the ideal gas law, providing a relationship between the number of atoms or molecules in a system and its physical properties.
– Faraday’s Laws of electrolysis in 1834 established the constant chemical decomposing action of a current.
– Krönig and Clausius independently derived the ideal gas law from kinetic theory in 1856.
– Loschmidt estimated the size of gas molecules and the number of molecules in a given volume of gas in 1865.
– vant Hoff demonstrated similarities between dilute solutions and ideal gases in 1886.
– The term ‘mole’ was first used by Ostwald in a university textbook in 1893.
– The mole was recommended for inclusion in the International System of Units (SI) in 1968.
– In 1972, the mole was approved as the SI base unit of amount of substance.
– In 2019, the mole was redefined in the SI as the amount of substance containing 6.02214076×10 specified elementary entities.
– The development of mass spectrometry in 1886 supported the concept of atomic and molecular mass and provided a tool for direct relative measurement. Source: https://en.wikipedia.org/wiki/Amount_of_substance
In chemistry, the amount of substance (symbol n) in a given sample of matter is defined as a ratio (n = N/NA) between the number of elementary entities (N) and the Avogadro constant (NA). The entities are usually molecules, atoms, or ions of a specified kind. The particular substance sampled may be specified using a subscript, e.g., the amount of sodium chloride (NaCl) would be denoted as nNaCl. The unit of amount of substance in the International System of Units is the mole (symbol: mol), a base unit. Since 2019, the value of the Avogadro constant NA is defined to be exactly 6.02214076×1023 mol−1. Sometimes, the amount of substance is referred to as the chemical amount or, informally, as the "number of moles" in a given sample of matter.
| Amount of substance | |
|---|---|
Common symbols | n |
| SI unit | mol |
| Dimension | |
