Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%), 87 (abundance of 7.0%), and 88 (abundance of 82.6%).
Calculate the atomic mass of
strontium.

M = 87.71 amu

Explanation:

The masses of 4 isotopes of Strontium with abundance is as follows :

84 (abundance 0.50%), 86 (abundance of  9.9%), 87 (abundance of 7.0%), and 88 (abundance of 82.6%). Let M is the atomic mass of  Strontium. So,

So, the  atomic mass of  Strontium is 87.71 amu.

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there are two significant figures.