# Molar mass of Arsenic (As)

1. Dec 31, 2015

### 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:
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:
• 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. Dec 31, 2015

Staff Emeritus
How many neutrons does arsenic have?

3. Dec 31, 2015

### phy9

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..

4. Dec 31, 2015

Mass defect.