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Molar Mass
In chemistry , the MOLAR MASS _M_ is a physical property defined as the mass of a given substance (chemical element or chemical compound ) divided by the amount of substance . The base SI unit
SI unit
for molar mass is kg /mol . However, for historical reasons, molar masses are almost always expressed in g/mol. As an example, the molar mass of water: _M_(H2O) ≈ 6998180000000000000♠18 g/mol. CONTENTS * 1 Molar masses of elements * 2 Molar masses of compounds * 3 Average molar mass of mixtures * 4 Related quantities * 4.1 Molecular mass * 4.2 DNA
DNA
synthesis usage * 5 Precision and uncertainties * 6 Measurement * 6.1 Vapour density * 6.2 Freezing-point depression
Freezing-point depression
* 6.3 Boiling-point elevation
Boiling-point elevation
* 7 References * 8 External links MOLAR MASSES OF ELEMENTS Main articles: Relative atomic mass
Relative atomic mass
and Standard atomic weight
Standard atomic weight
The molar mass of atoms of an element is given by the Standard atomic weight of the element multiplied by the molar mass constant , _M_ u = 1 × 10−3 kg/mol = 1 g/mol: _M_(H) = 1.007 97(7) × 1 g/mol = 1.007 97(7) g/mol _M_(S) = 32.065(5) × 1 g/mol = 32.065(5) g/mol _M_(Cl) = 35.453(2) × 1 g/mol = 35.453(2) g/mol _M_(Fe) = 55.845(2) × 1 g/mol = 55.845(2) g/mol
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Si Unit
The INTERNATIONAL SYSTEM OF UNITS (abbreviated as SI, from the French _Système internationale (d'unités)_) is the modern form of the metric system , and is the most widely used system of measurement . It comprises a coherent system of units of measurement built on seven base units . The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre-kilogram-second system of units (MKS) rather than any variant of the centimetre–gram–second system (CGS). SI is intended to be an evolving system, so prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The 24th and 25th General Conferences on Weights and Measures (CGPM) in 2011 and 2014, for example, discussed a proposal to change the definition of the kilogram , linking it to an invariant of nature rather than to the mass of a material artefact, thereby ensuring long-term stability. The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems and the lack of coordination between the various disciplines that used them
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Chemistry
CHEMISTRY is a branch of physical science that studies the composition, structure, properties and change of matter . Chemistry includes topics such as the properties of individual atoms , how atoms form chemical bonds to create chemical compounds , the interactions of substances through intermolecular forces that give matter its general properties, and the interactions between substances through chemical reactions to form different substances. Chemistry is sometimes called the central science because it bridges other natural sciences , including physics , geology and biology . For the differences between chemistry and physics see comparison of chemistry and physics . The history of chemistry can be traced to alchemy , which had been practiced for several millennia in various parts of the world
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Mass
In physics , MASS is a property of a physical body . It is the measure of an object's resistance to acceleration (a change in its state of motion ) when a net force is applied. It also determines the strength of its mutual gravitational attraction to other bodies. The basic SI unit of mass is the kilogram (kg). Mass is not the same as weight , even though mass is often determined by measuring the object's weight using a spring scale , rather than comparing it directly with known masses . An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force. In Newtonian physics , mass can be generalized as the amount of matter in an object. However, at very high speeds, special relativity postulates that energy becomes a significant additional source of mass. Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction. In addition, "matter" is a loosely defined term in science, and thus cannot be precisely measured. There are several distinct phenomena which can be used to measure mass
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Chemical Element
A CHEMICAL ELEMENT or ELEMENT is a species of atoms having the same number of protons in their atomic nuclei (i.e. the same atomic number , or _Z_). There are 118 elements that have been identified, of which the first 94 occur naturally on Earth
Earth
with the remaining 24 being synthetic elements . There are 80 elements that have at least one stable isotope and 38 that have exclusively radioactive isotopes , which decay over time into other elements. Iron
Iron
is the most abundant element (by mass ) making up Earth, while oxygen is the most common element in the Earth\'s crust . Chemical elements constitute all of the ordinary matter of the universe. However astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter ; the composition of this is unknown, but it is not composed of chemical elements. The two lightest elements, hydrogen and helium , were mostly formed in the Big Bang and are the most common elements in the universe. The next three elements (lithium , beryllium and boron ) were formed mostly by cosmic ray spallation , and are thus rarer than heavier elements. Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis . The high abundance of oxygen, silicon , and iron on Earth
Earth
reflects their common production in such stars
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Chemical Compound
A CHEMICAL COMPOUND (or just COMPOUND if used in the context of chemistry ) is an entity consisting of two or more atoms , at least two from different chemical elements , which associate via chemical bonds . There are four types of compounds, depending on how the constituent atoms are held together: molecules held together by covalent bonds , ionic compounds held together by ionic bonds , intermetallic compounds held together by metallic bonds , and certain complexes held together by coordinate covalent bonds . Many chemical compounds have a unique numerical identifier assigned by the Chemical Abstracts Service (CAS): its CAS number . A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using the standard abbreviations for the chemical elements, and subscripts to indicate the number of atoms involved. For example, water is composed of two hydrogen atoms bonded to one oxygen atom: the chemical formula is H2O. A compound can be converted to a different chemical composition by interaction with a second chemical compound via a chemical reaction . In this process, bonds between atoms are broken in both of the interacting compounds, and then bonds are reformed so that new associations are made between atoms
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Amount Of Substance
AMOUNT OF SUBSTANCE is a standards-defined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as CHEMICAL AMOUNT. The International System of Units (SI) defines the amount of substance to be proportional to the number of elementary entities present. The SI unit for amount of substance is the mole . It has the unit symbol mol. The proportionality constant is the inverse of the Avogadro constant
Avogadro constant
. The mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12g of the isotope carbon-12 . This number is called Avogadro's number and has the value 7023602214085700000♠6.022140857(74)×1023. It is the numerical value of the Avogadro constant
Avogadro constant
which has the unit 1/mol, and relates the molar mass of an amount of substance to its mass. Therefore, the amount of substance of a sample is calculated as the sample mass divided by the molar mass of the substance. Amount of substance
Amount of substance
appears in thermodynamic relations such as the ideal gas law , and in stoichiometric relations between reacting molecules as in the law of multiple proportions . Another unit of amount of substance in use in chemical engineering in the United States is the pound-mole , having the symbol lb-mol
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Kilogram
The KILOGRAM or KILOGRAMME ( SI unit symbol: KG) is the base unit of mass in the International System of Units (SI) (the Metric system ) and is defined as being equal to the mass of the _International Prototype of the Kilogram_ (IPK, also known as "Le Grand K" or "Big K"). The avoirdupois (or _international_) pound , used in both the imperial and US customary systems, is defined as exactly 6999453592370000000♠0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are also defined in terms of the kilogram, making the IPK the primary standard for virtually all units of mass on Earth
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Mole (unit)
The MOLE is the unit of measurement in the International System of Units (SI) for amount of substance . The unit is defined as the amount of a chemical substance that contains as many representative particles, e.g., atoms , molecules , ions , electrons , or photons , as there are atoms in 12 grams of carbon-12 (12C), the isotope of carbon with standard atomic weight 12 by definition. This number is expressed by the Avogadro constant
Avogadro constant
, which has a value of 7023602214085700000♠6.022140857×1023 mol−1, which is just over 602 sextillion (thousand million million million ). The mole is one of the base units of the SI, and has the unit symbol MOL. The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2 H2 + O2 → 2 H2O implies that 2 moles of dihydrogen (H2) and 1 mole of dioxygen (O2) react to form 2 moles of water (H2O). The mole may also be used to express the number of atoms, ions, or other elementary entities in a given sample of any substance. The concentration of a solution is commonly expressed by its molarity , defined as the number of moles of the dissolved substance per litre of solution. The number of molecules per mole is known as Avogadro\'s constant , and is defined such that the mass of one mole of a substance, expressed in grams, is equal to the mean relative molecular mass of the substance
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Relative Atomic Mass
RELATIVE ATOMIC MASS (symbol: _A_r) is a dimensionless (number only) physical quantity . In its modern definition, it is the _ratio of_ the average mass of atoms of an element in a given sample _to_ one unified atomic mass unit . The unified atomic mass unit, symbol u, is defined being  1⁄12 of the mass of a carbon-12 atom. The mass of atoms can vary (between atoms of the same element), due to the presence of various isotopes of that element. Since both values in the ratio are expressed in the same unit (u), the resulting value is dimensionless; hence the value is _relative_. Within one source (sample), it is a straight average over the individual atom weights (isotopes) present. Between sources, the atomic weight can vary when the source's origin (radioactive history) resulted in different isotopic concentrations. These differences are real and measurable, and can be used to identify a sample to its origin. For example, a sample of elemental carbon from volcanic methane will have a different relative atomic mass than one collected from plant or animal tissues. The well-known STANDARD ATOMIC WEIGHT , or ATOMIC WEIGHT, is a specific usage of _relative atomic mass_: it is the relative atomic mass, with the sources being terrestrial (taken from Earth ). The standard atomic weights are reprinted in a wide variety of textbooks, commercial catalogues, and periodic table wall charts. They are what chemists loosely call "atomic weights"
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Standard Atomic Weight
The STANDARD ATOMIC WEIGHT (_A_r, standard) or ATOMIC WEIGHT is a physical quantity for a chemical element, expressed as a relative atomic mass (_A_r). It is specified by (restricted to) the IUPAC ( CIAAW ) definition of natural, stable, terrestrial sources. Because of this practical definition, the value is widely used as 'the' atomic weight for real life substances. For example, in pharmaceuticals and scientific research. Out of the 118 known chemical elements, 84 are stable and have this Earth-environment based value. Typically, such a value is, for example helium: _A_r, standard(He) = 7000400260200000000♠4.002602(2). The "(2)" indicates the uncertainty in the last digit shown, to read 7000400260200000000♠4.002602 ±6994200000000000000♠0.000002. IUPAC also publishes _abridged values_, rounded to five significant figures. For helium, _A_r, abridged(He) = 7000400260000000000♠4.0026. For twelve elements the samples diverge on this value, because their sample sources have had a different decay history. For example, thallium (Tl) in sedimentary rocks has a different isotopic composition than in igneous rocks and volcanic gases. For these elements, the standard atomic weight is noted as an interval: _A_r, standard(Tl) = . With such an interval, for less requiring situations, IUPAC also publishes an _conventional value_. For thallium, _A_r, conventional(Tl) = 7002204380000000000♠204.38
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Atom
Atom, from the Greek word atomos, which means indivisible, was first conceived around 2,400 years ago by a Greek man named Democritus . An ATOM is the smallest constituent unit of ordinary matter that has the properties of a chemical element . Every solid , liquid , gas , and plasma is composed of neutral or ionized atoms. Atoms are very small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale ). Atoms are small enough that attempting to predict their behavior using classical physics – as if they were billiard balls, for example – gives noticeably incorrect predictions due to quantum effects . Through the development of physics, atomic models have incorporated quantum principles to better explain and predict the behavior. Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and typically a similar number of neutrons . Protons and neutrons are called nucleons . More than 99.94% of an atom's mass is in the nucleus. The protons have a positive electric charge , the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, that atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion
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Molar Mass Constant
The MOLAR MASS CONSTANT, symbol _M_u, is a physical constant which relates relative atomic mass and molar mass . Its value is defined to be 1 g/mol in SI units. The molar mass constant is important in writing dimensionally correct equations. It is common to see phrases such as _The molar mass of an element is the atomic weight in grams per mole._ However, atomic weight, i.e., relative atomic mass, is a dimensionless quantity, and cannot take the units of grams per mole. Formally, the operation is the multiplication by a constant which has the value 1 g/mol, that is the molar mass constant. The molar mass constant is unusual (but not unique) among physical constants by having an exactly defined value rather than being measured experimentally. It is fixed by the definitions of the mole and of relative atomic mass . From the definition of the mole, the molar mass of carbon 12 is exactly 12 g/mol. From the definition of relative atomic mass, the relative atomic mass of carbon 12, that is the atomic weight of a sample of pure carbon 12, is exactly 12
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Dimension
In physics and mathematics , the DIMENSION of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it – for example, the point at 5 on a number line. A surface such as a plane or the surface of a cylinder or sphere has a dimension of two because two coordinates are needed to specify a point on it – for example, both a latitude and longitude are required to locate a point on the surface of a sphere. The inside of a cube , a cylinder or a sphere is three-dimensional because three coordinates are needed to locate a point within these spaces. In classical mechanics , space and time are different categories and refer to absolute space and time . That conception of the world is a four-dimensional space but not the one that was found necessary to describe electromagnetism . The four dimensions of spacetime consist of events that are not absolutely defined spatially and temporally, but rather are known relative to the motion of an observer . Minkowski space first approximates the universe without gravity ; the pseudo-Riemannian manifolds of general relativity describe spacetime with matter and gravity
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Molecule
A MOLECULE is an electrically neutral group of two or more atoms held together by chemical bonds . Molecules are distinguished from ions by their lack of electrical charge . However, in quantum physics , organic chemistry , and biochemistry , the term _molecule_ is often used less strictly, also being applied to polyatomic ions . In the kinetic theory of gases , the term _molecule_ is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are in fact monoatomic molecules. A molecule may be homonuclear , that is, it consists of atoms of one chemical element , as with oxygen (O2); or it may be heteronuclear , a chemical compound composed of more than one element, as with water (H2O). Atoms and complexes connected by non-covalent interactions , such as hydrogen bonds or ionic bonds , are generally not considered single molecules. Molecules as components of matter are common in organic substances (and therefore biochemistry). They also make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the crust , mantle , and core of the Earth , contain many chemical bonds, but are _not_ made of identifiable molecules
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Hydrogen
HYDROGEN is a chemical element with symbol H and atomic number 1. With a standard atomic weight of circa 7000100800000000000♠1.008, hydrogen is the lightest element on the periodic table . Its monatomic form (H) is the most abundant chemical substance in the Universe , constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state . The most common isotope of hydrogen, termed _protium_ (name rarely used, symbol 1H), has one proton and no neutrons . The universal emergence of atomic hydrogen first occurred during the recombination epoch . At standard temperature and pressure , hydrogen is a colorless , odorless , tasteless , non-toxic, nonmetallic , highly combustible diatomic gas with the molecular formula H2. Since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds . Hydrogen plays a particularly important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds , hydrogen can take the form of a negative charge (i.e., anion ) when it is known as a hydride , or as a positively charged (i.e., cation ) species denoted by the symbol H+. The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex
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