1) You need the chemical formula: di-sulfur means there are two atoms of sulfur and deca-fluoride means there are 10 atoms of fluor in the formula.
S2F10
2) Now calculate the molar mass, using the atomic mass of each element
S: 232g/mol = 64g/mol
F: 1019g/mol = 190 g/mol
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S2F10 255g/mol
3) Now divide the mass of the sample by its molar mass, to get the number of moles that contains the sample
125 g / 255g/mol = 0.49 mol
4) Now use the subscripts for every element and Avogadro's number:
S atoms: 2 [mol/mol-of-compound] 6.02210^23 atoms / mol 0.49 mol-of-compound = 5.9010^23 atoms of S
F atoms: 10 [mol/mol-of-compound]6.02210^23 atoms / mol 0.49 mol-of-compound = 29.510^23 atoms of F
Total number of atoms = 5.9010^23 + 29.510^23 = 35.4 * 10^23 atoms
