Why Does My Calculated Molar Mass of Arsenic Differ from the Standard Value?

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The calculated molar mass of arsenic (As) differs from the standard value of 74.92 g/mol found on ptable.com due to the method of calculation used. The calculations initially focused on protons and neutrons, with the user noting that electrons were negligible in their impact. The user determined that arsenic has 42 neutrons, leading to a revised calculation that yielded a molar mass of approximately 75.60 g/mol. This discrepancy highlights the importance of considering mass defect in atomic mass calculations. Ultimately, the difference arises from the simplifications made in the calculations and the inherent complexities of atomic mass.
phy9
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Hi,

I'm trying to calculate the molar mass of As, tough it differs from the value that is written in http://www.ptable.com.

The molar mass of Arsenic (As) according to 'ptable.com' is 74.92 g/mol.

I tried to calculate the molar mass with python:
Python:
>>> Nav = 6.022140857E23
>>> mp = 1.6726219E-27
>>> mp *= 1000 # in grams
>>> 2*33*(mp*Nav)
66.48024690319959
>>> me = (9.10938356E-31)*1000
>>> me
9.10938356e-28
>>> 2*33*(mp*Nav) + 33*me*Nav
66.49835004020278
Explanation:
  • Nav is Avogadro constant
  • mp is proton mass, in grams
  • me is electron mass, in grams
  • The calculation "2*33*(mp*Nav)" considers neutrons but neglects electrons.
    The number 33 is the atomic number of (As), which indicates the number of protons.
  • I also tried to consider electrons but you can see that they don't matter.
What is the problem with my calculation?
 
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phy9 said:
The calculation "2*33*(mp*Nav)" considers neutrons but neglects electrons.

How many neutrons does arsenic have?
 
Vanadium 50 said:
How many neutrons does arsenic have?
Thanks..
Apparently 42.

Python:
>>> 33*(mp*Nav) + 42*mn*Nav
75.60404990520709
while mn is mass of a neutron.
Though it is a bit higher than the value in ptable.com..
 
phy9 said:
Though it is a bit higher than the value in ptable.com..

Mass defect.
 
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