The two naturally occurring isotopes of antimony are Sb-121 and Sb-123.
The table below shows the atomic mass and percent natural abundance for
Antimony and sulfur are both found in the mineral stibnite, Sb2S3. To
obtain antimony, stibnite is roasted (heated in air), producing oxides
of antimony and sulfur. The unbalanced equation below represents one of
the reactions that occurs during the roasting.
Sb2S3(s) + O2(g) → Sb2O3(s)
66 Determine the percent composition by mass of antimony in stibnite
(gram-formula mass = 340. g/mol). 
67 In the space in your answer booklet, show a correct numerical setup
for calculating the atomic mass of antimony. 
Answer-->(120.90*57) + (122.90*43)/100
68 Complete the balancing of the equation in your answer booklet for the
roasting of stibnite, using the smallest whole-number coefficients.