The textbook context directly lists these three reactions as Question 2. For part (i), hydrogen and chlorine combine in a 1:1 ratio to form hydrogen chloride, requiring coefficient 2 for HCl to balance. For part (ii), this is a double displacement reaction where 3 moles of barium chloride react with 1 mole of aluminium sulphate to produce 3 moles of barium sulphate and 2 moles of aluminium chloride. For part (iii), sodium displaces hydrogen from water in a displacement reaction, requiring 2 moles each of sodium and water to produce 2 moles of sodium hydroxide and 1 mole of hydrogen gas.