When thinking about stoichiometry problems, the first thing that should be done is to write a reaction. In this case, HF and Sn are reacting, thus they will be reactants. Then you follow the reaction guidelines for single replacement reactions, giving you the following reaction:
Sn+HF=SnF2+H2
However, there is an unequal amount of Hydrogen and Fluorine on both sides of the reaction, which will require us to balance the equation. By putting 2 moles of HF, there will be 2 H and 2 F on both sides.
Sn+2HF=SnF2+H2
Here, there is 1 mole of Sn and 2 moles of HF reacting. If there are 10 moles of Sn, there should be 20 moles of HF that will react, since it is a 1:2 ratio.
