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  • A Walk Through Time Featured Article A Walk Through Time A Walk Through Time

    The Evolution of Time Measurement through the Ages. A Walk Through Time Ancient Calendars Celestial bodies — the Sun, Moon, planets, and stars — have... More »

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International Union of Pure and Applied Chemistry Last Updated on 2012-04-08 00:00:00 The International Union of Pure and Applied Chemistry (IUPAC) is a non-governmental scientific organization serving to advance all aspects of the chemical sciences and to contribute to the international application of chemistry by scientists, engineers and others. It is an association of organizations, called "National Adhering Organizations", which represent the chemists of different member countries. As of July 2009, there were 54 "National Adhering Organizations" plus some other countries that are linked to IUPAC as "Associate National Adhering Organizations".[1][2] The U. S. National Academy of Sciences is the U.S. National Committee for the International Union of Pure and Applied Chemistry (i.e., the IUPAC's National Adhering Organization for the United States.[3] The international headquarters office of the IUPAC is located in Zurich,... More »
Concentration expressions and notations Last Updated on 2012-01-26 00:00:00 In chemistry and other sciences, engineering and in fairly common usage, concentration is the measure of how much of a given substance there is in a given mixture of substances. Concentration expressions and notations There are many different notations and quantitative expressions of concentration.[1] The most commonly used expressions are discussed below: Mole fraction and mole percent The mole fraction is a measure of the concentration of a component substance in a mixture of substances. It is defined as the number of moles of a component substance in a mixture divided by the total number of moles of the mixture.[2][3] The mole percent  (also referred to as the molar percent) is usually denoted by mole % and is equal to 100 times the mole fraction. Mass fraction and mass percent The mass fraction is also a measure of the concentration of a component... More »
National Institute of Standards and Technology Last Updated on 2012-01-21 00:00:00 The National Institute of Standards and Technology (NIST) is a United States federal agency within the U.S. Department of Commerce.[1] The institute was founded in 1901 with the aim of advancing measurement science, standards, and technology. NIST was known between 1901–1988 as the National Bureau of Standards (NBS). NIST has an operating budget of about $1,600,000,000[2] and operates in two locations: Gaithersburg, Maryland and Boulder, Colorado. NIST employs a staff of about 2,900 scientists, engineers, technicians, and support and administrative personnel. About 2,600 associates and facility users from academia, industry and other government agencies complement the staff.[3] History Article 1, Section 8 of the United States Constitution grants the U.S. Congress the power to "To coin money, regulate the value thereof, and of foreign coin, and fix... More »
Molar gas constant Last Updated on 2012-01-17 00:00:00 The molar gas constant (also known as the universal or ideal gas constant and designated by the symbol R ) is an important physical constant which appears in many of the fundamental equations in physics, chemistry and engineering, such as the ideal gas law, other equations of state and the Nernst equation. R is equivalent to the Boltzmann constant (designated kB) times Avogadro's constant (designated NA) and thus: R = kBNA. Currently the most accurate value as published by the National Institute of Standards and Technology (NIST) is:[1] R = 8.3144621 J · K–1 · mol–1 A number of values of R in other units are provided in the adjacent table. The gas constant occurs in the ideal gas law as follows: PV = nRT or as: PVm = RT where: P  is the absolute pressure of the gas T  is the absolute temperature of the... More »
A Walk Through Time Last Updated on 2011-12-21 00:00:00 The Evolution of Time Measurement through the Ages. A Walk Through Time Ancient Calendars Celestial bodies — the Sun, Moon, planets, and stars — have provided us a reference for measuring the passage of time throughout our existence. Ancient civilizations relied upon the apparent motion of these bodies through the sky to determine seasons, months, and years. We know little about the details of timekeeping in prehistoric eras, but wherever we turn up records and artifacts, we usually discover that in every culture, some people were preoccupied with measuring and recording the passage of time. Ice-age hunters in Europe over 20,000 years ago scratched lines and gouged holes in sticks and bones, possibly counting the days between phases of the moon. Five thousand years ago, Sumerians in the Tigris-Euphrates valley in today's Iraq had a calendar that... More »