Avogadro constant determination

Now we can determine the mass per A1 atom, and a value for the Avogadro constant. 

The number of elementary entities in 1 mol is an experimentally determined quantity, and is called the Avogadro constant L, which has the value 6.022 x 1023 mol-1. The Avogadro constant is also (incorrectly) called the Avogadro number . It is... 

A powder diffraction pattern establishes that silver crystallizes in a face-centred cubic unit cell. The 111 reflection is observed at 0=19.1°, using Cu-Ka radiation. Determine the unit cell length, a. If the density of silver is 10.5x10 kg m and Z=4, calculate the value of the Avogadro constant. (The atomic mass of silver is 107.9.)... 

AVOGADRO CONSTANT. The number of molecules contained in one mole or gram-molecular weight of a substance. The most recent value is 6.0220943 x lO23 6.3 x 1017. In measurements made by scientists at the National Bureau of Standards (Gaithersburg, Maryland) and announced in late 1974. the uncertainty (as compared with previous determinations) of the number was reduced hy a factor of 30. 

The molar masses of elements are determined by using mass spectrometry to measure the masses of the individual isotopes and their abundances. The mass per mole of atoms is the mass of an individual atom multiplied by the Avogadro constant (the number of atoms per mole). However, there is a complication. Most elements occur in nature as a mixture of isotopes we saw in Section B, for instance, that neon occurs as three isotopes, each with a different mass. In chemistry, we almost always deal with natural samples of elements, which have the natural abundance of isotopes. So, we need the average molar mass, the molar mass calculated by taking into account the masses of the isotopes and their relative abundances in typical samples. All molar masses quoted in this text refer to these average values. Their values are given in Appendix 2D. They are also included in the periodic table inside the front cover and in the alphabetical list of elements inside the back cover. 

Determination of the amount of substance is thus in direct relation to basic units of the SI system and does not need a RM for comparison. The Faraday constant is one of the fundamental constants (it can be expressed as the product of the electron charge and the Avogadro constant). It enables the attainment of high precision and accuracy and is independent of the atomic weights of the elements in the sample. Its drawback is lower selectivity, a feature common to titration methods. This makes coulometry especially suitable for determination of relatively pure substances used as standards by other (relative) methods. The Faraday constant has been proposed as an ultimate standard in chemistry [3],... 

The 1998 adjustment of the values of the fundamental physical constants has been carried out by the authors under the auspices of the CODATA Task Group on Fundamental Constants [1,2]. The purpose of the adjustment is to determine best values of various fundamental constants such as the fine-structure constant, Rydberg constant, Avogadro constant, Planck constant, electron mass, muon mass, as well as many others, that provide the greatest consistency among the most critical experiments based on relationships derived from condensed matter theory and quantum electrodynamics (QED) theory. The 1998 CODATA recommended values of the constants also may be found on the Web at physics.nist.gov/constants. 

The Avogadro constant is an experimentally determined quantity. Chemists continually devise more accurate methods to determine how many atoms are in exactly 12 g of carbon-12. This means that the accepted value has changed slightly over the years since it was first defined. 

Chemists have devised various ways to determine the Avogadro constant. To learn more about how this constant has been found in the past and how it is found today, go to the web site above. Go to Science Resources, then to Chemistry 11 to find out where to go next. What are some methods that chemists have used to determine the number of particles in a mole How has the accepted value of the Avogadro constant changed over the years ... 

You can use what you now know about the mole to carry out calculations involving molar mass and the Avogadro constant. One mole of any compound or element contains 6.02 x 1023 particles. The compound or element has a mass, in grams, that is determined from the periodic table. 

Use the definition of the Avogadro constant to explain why its value must be determined by experiment. 

Avogadro constant (ATa) the experimentally-determined number of particles in 1 mol of a substance the currently accepted value is 6.022 141 99 x 1023 (5.2) Avogadro s hypothesis equal volumes of gases, at the same temperature and pressure, contain the same number of particles (12.1)... 

Radium-224 is radioactive and decays by emitting an a-particle. (a) Write an equation for this process, (b) The decay of radium-224 produces helium gas. Rutherford and Geiger determined that a-particles were emitted from ggRa at a rate of 7.65 x lO s moU, and that this corresponded to a rate of helium production of 2.90 X dm s at 273 K, 1 bar. If 1 mole of helium occupies 22.7 dm (273 K, 1 bar), estimate a value for the Avogadro constant. 

Determination of the Avogadro constant and molar volume from physical measurement... 

The mass standard has been defined as the mass of a platinum-iridium cylinder (Figure 1.38) that is stored in an air-tight container at the International Bureau of Weights and Measures (BIPM) in Sevres, France. An alloy of platinum and iridium was chosen for this standard because it is highly resistant to chemical attack and its mass will not change for a very long time. Scientists are in search of a new standard of mass via accurate determination of the Avogadro constant. 

If it can be determined how many of these charges are needed to make up the number of coulombs in 1 Faraday, then you will know how many electrons there are in a mole of electrons, that is, the Avogadro constant. 

The Avogadro constant can be calculated from dividing the charge on the electron by the experimentally determined value for the Faraday constant ... 

A current of 3.00 A was passed for 30 minutes through molten lead(ii) bromide. Lead of mass 5.60 grams was obtained. Determine the value of the Avogadro constant. 

A value for the Avogadro constant can be determined by an electrolytic method using the relationship F=Le. 

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