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Molar mass of Arsenic (As)

  1. Dec 31, 2015 #1
    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:
    Code (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?
     
  2. jcsd
  3. Dec 31, 2015 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    How many neutrons does arsenic have?
     
  4. Dec 31, 2015 #3
    Thanks..
    Apparently 42.

    Code (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..
     
  5. Dec 31, 2015 #4
    Mass defect.
     
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