1 mol of XCl3 contains 3 mols of Cl.
But we have 23.78g of XCl3 contains 4.81g of X ,Thus the remaining weight corresponds to the mass of the Cl present in the given compound,Hence the mass of the Cl
= 23.78 - 4.81
= 18.97g
we know that 1mol of Cl = 35g
therefore 18.97g of Cl = 0.542mol
1mol of XCl3 contains 3mol of Cl
∴0.542 mol of Cl present in mole of XCl3 = 0.542/3
= 0.18 mol
0.18mol of X = 4.81g
∴ 1 mol of X = 26.72g
This is the atomic weight of the X which indicates that the atom may be "Al" in th periodic table.