Final Answer:
a. The metal reacting with 10% HCl to form solution X is Aluminum (Al).
b. The crystal formula for the hydrated crystals obtained by reacting 11.2g of metal A with 10% H₂SO₄ and concentrating the solution to get 66.4g of crystals is

Step-by-step explanation:
a. The reaction of metal A with 10% HCl suggests that it is Aluminum (Al) because aluminum reacts with hydrochloric acid to produce hydrogen gas and form the corresponding salt.
b. The reaction of 11.2g of Aluminum (A) with 10% H₂SO₄ yields Aluminum sulfate,
, and hydrogen gas. By concentrating the solution, hydrated crystals are formed. The crystal formula is determined by considering the number of water molecules associated with the salt. In this case, there are 18 water molecules, so the crystal formula is
). The molar mass of the crystal can be calculated to verify its weight, ensuring it matches the given value of 66.4g.