The molar mass of hf is 20.01 g/mol. how many moles of sn are required to react with 40 g of hf?

You are given the molar mass of HF equals 20.01 grams per mole and the mass of HF equals 40 grams. You are required to get the number of moles of Sn in the reaction. The balanced chemical reaction is Sn(s) + HF(g) -> SnF2(s) + H2(g). Note that one mole of Sn is required to react with one mole of HF. So use the given mass of HF to get the moles of Sn,  

40 g HF(1 mol HF/20.01 g HF)(1 mol Sn/1 mol HF) = 1.999 or 2 moles of Sn  

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.