Atomic Weight/Molar Mass:       

        Isotopes and Mass Spectrometry:  http://whyfiles.org/083isotope/2.html

Periodic Table with linked Isotopes: http://ie.lbl.gov/education/isotopes.htm
                or http://nucleardata.nuclear.lu.se/nucleardata/toi/perchart.htm

    Molecular Weight, Molar Concentration Calculators: http://www.martindalecenter.com/Calculators3_2_Bio.html#BIO-MOLAR