help please. In 1961 the mlar atomic mass of carbon 12 was defined to be exactly 12g/mol (a value agreed upon by chemists and physicists) and the molar atomic masses of all the elements were re-evaluated according to this new standard.
This made the re-evaluated molar mass of oxygen-16 become 15.994915g/mol. Avogadro's number is defined as the number of atoms in the molar atomic mass of an element. The present accpted value for Avogadros' number is 6.0221367x10^23/mol.
What was the value o Avogadro's number used by physicists before 1961?

Question

help please.  In 1961 the mlar atomic mass of carbon 12 was defined to be exactly 12g/mol (a value agreed upon by chemists and physicists) and the molar atomic masses of all the elements were re-evaluated according to this new standard. 
This made the re-evaluated molar mass of oxygen-16 become 15.994915g/mol. Avogadro's number is defined as the number of atoms in the molar atomic mass of an element. The present accpted value for Avogadros' number is 6.0221367x10^23/mol. 
What was the value o Avogadro's number used by physicists before 1961?

anonymous · Answer

The first person to have calculated the number of molecules in any mass of substance was Josef Loschmidt, (1821-1895), who calculated the number of molecules in one cubic centimeter of gaseous substance under ordinary conditions of temperature of pressure, to be somewhere around 2.6 x 10^19 molecules. This is usually known as "Loschmidt's Constant." 

The best modern values for what we now call "Avogadro's Number" are the result of the x-ray diffraction measurement of lattice distances in metals and salts. Today's best experimental value of 6.022 4199 x 10^23 mol (obtained from NIST web site) is the best average for measurements using the best methods available. The experiments are often very difficult to carry out. That the number today has 8 significant figures is a testament to the quality of modern experimental methods. Hope it helps ya buddy!