For problems like this, the first information that we will be needing is the balanced chemical reaction. This will tell us the involved reactants and the products generated and how much of each is consumed and produced. Since we are given the moles of HCl and we are asked for the moles of the H2S gas, then this would be a very straight forward problem. This will only need the mole ratio of both components which can be extrapolated form the chemical reaction equation.
2.56 moles HCl * (1mole H2S/ 2 moles HCl) = 1.28 moles H2S
