The chemical reaction that is followed when hydrogen sulfide and sulfur dioxide react when they are mixed is: 2H2S (g) + SO2 (g) ---> 2H2O (l) + 3S (s)
16 g or hydrogen sulfide and 20 g of sulfur dioxide take part in the reaction. The molar mass of hydrogen sulfide is 34 and that of sulfur dioxide is 64. 16 g of hydrogen sulfide is 0.4705 moles and 20 g of sulfur dioxide is 0.3125 moles.
As 2 moles of hydrogen sulfide react with one mole if sulfur dioxide is the reaction, only 0.2352 moles of sulfur dioxide can react. This leaves 0.07720 moles of sulfur dioxide in the unreacted form. The mass of this is 0.0772*64 = 4.94 g.