To find the mass of a mole of a compound, scientists add together the???

Question

Since this is not my area of expertise, I searched Google under the key words "find mass mole" to get these possible sources:

http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson92.htm
http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson91.htm
http://wind.cc.whecn.edu/~mechalke/chapter9/chapter9.htm

I hope this helps. Thanks for asking.