25 mL x (1L/1000 mL) x (4 mol HCl/1L) x (1 mol Zn/2 mol HCl) x (65.38 g Zn/1 mol Zn) = 3.269g = ~ 3.3g of Zn
Parts explained
25 mL x (1L/1000 mL) x (4 mol HCl/1L) = 0.1 mol of HCl that reacts with Zn. Note: (1L/1000mL) is conversion factor to change mL to L
25 mL x (1L/1000 mL) x (4 mol HCl/1L) x (1 mol Zn/2 mol HCl) = 0.05 mol of Zn that reacts completely with 0.1 mol of HCl. Note: From the balanced equation, 2 mol of HCl reacts with 1 mol Zn, thus we use (1 mol Zn/2 mol HCl) as conversion factor
(65.38 g Zn/1 mol Zn) = Molar mass of Zn, used to convert moles to grams.