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: 2*32g/mol = 64g/mol
F: 10*19g/mol = 190 g/mol
----------------
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] *6.022*10^23 atoms / mol * 0.49 mol-of-compound = 5.90*10^23 atoms of S
F atoms: 10 [mol/mol-of-compound]*6.022*10^23 atoms / mol * 0.49 mol-of-compound = 29.5*10^23 atoms of F
Total number of atoms = 5.90*10^23 + 29.5*10^23 = 35.4 * 10^23 atoms
