prior to 1961 the molar Prior to 1961, the molar atomic mass of the oxygen-16 is
ID: 491942 • Letter: P
Question
prior to 1961 the molar Prior to 1961, the molar atomic mass of the oxygen-16 isotope was taken as exactly 16 g/mol by physicists. Chemists actually used a different value. In 1961 the molar atomic mass of carbon 12 was defined to be exactly 12 g/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 15.994915 g/mol. Avogadro's number is defined as the number of atoms in the molar atomic mass of an element. The present accepted value for Avogadro's number is 6.0221367 times 10^23/mol. What was the value of Avogadro's number used by physicists before 1961?Explanation / Answer
Ans. Given, Present accepted molar atomic mass of O = 15.994915 g/mol
Present accepted value of Avogadro number = 6.0221367 x 1023 / mol
Previous accepted molar atomic mass of O = 16 g/mol
Avogadro number defined as number of atoms in the molar atomic mass of the element.
Now, Using present accepted values-
Mass of 1 O-atom = present molar atomic mass/ Present Value of Avogadro number
= (15.994915 g/mol) / (6.0221367 x 1023 / mol )
= 2.6560199 x 10-23 g
## Tracking back to older values of Avogadro number.
So, we have,
Mass of 1 O-atoms = 2.6560199 x 10-23 g
Mass of 1 mol O-atom (Molar atomic mass of atom) = 16.0 g
Using “Avogadro number defined as number of atoms in the molar atomic mass of the element.” It’s assumed that mass of O-atoms remains constant, only value of Avogadro number has been modified in recent times.
Avogadro number = Molar atomic mass of O (1961, prior to) / Mass of 1 O-atom
= (16.0 g/ mol) / (2.6560199 x 10-23 g)
= 6.0240512 x 1023 / mol
Thus, accepted value of Avogadro number prior to 1961 = 6.0240512 x 10-23 / mol
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