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Answer 1 · 2016-04-15T18:41:35

#A_r=58.99#

The number of moles of a substance is given by:
#(mass (grams))/("Relative atomic mass", A_r)#

And one mole comprises (Avogadro's constant), #N_A =6.022 times 10^23# atoms / molecules.

So number of atoms is given by:
#(mass (grams)*N_A)/("Relative atomic mass", A_r)#

In this case

# ((16.26times 10^-3g)/A_r)*(6.022 times 10^23)=1.66 times 10^20#

Rearranging gives:

#A_r= (16.26times 10^-3g)*(6.022 times 10^23)/(1.66 times 10^20)#
so
#A_r=58.99#