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
