Strontium consists of four isotopes with masses and their percent abundance of 84. amu ( 0.5%), 86 amu (9.9%) , 87amu (7.0 %) , and 88. amu (82.6 %). calculate the relative average atomic mass of sr ?

The answer to your question is: 87.712 amu

Explanation:

Strontium  isotopes    1   mass    84 amu       abundance 0.5%

                                    2 mass     86 amu       abundance 9.9 %

                                   3 mass      87 amu        abundance 7%

                                    4 mass      88 amu       abundance 82.6%

average atomic mass = (84 x 0.005) + (86 x 0.099) + (87 x 0.07) + (88x0.826)

average atomic mass = 0.42 + 8.514 + 6.09 + 72.688

average atomic mass = 87.712 amu