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?