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

[phy9]

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:

>>> 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?

 

[Vanadium 50]

The calculation "2*33*(mp*Nav)" considers neutrons but neglects electrons.

How many neutrons does arsenic have?

 

[phy9]

How many neutrons does arsenic have?

Thanks..
Apparently 42.

>>> 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..

 

[DrStupid]

Though it is a bit higher than the value in ptable.com..

Mass defect.