The balanced chemical reaction is written as:
2XI3 + 3Cl2 ---> 2XCl3 + 3I2
To determine the atomic weight of element X, we need to know make a relation between the mass of the reactant XI3 and the mass of the product XCl3. In this case, we write equations that would equate to the mole of X in that particular substance. We do as follows:
0.5500 g XI3 ( 1 mol / x + 380.7 g XI3 ) ( 1 mol X / 1 mol XI3 ) = 0.2596 g XCl3 ( 1 mol / x + 106.35 g XCl3 ) ( 1 mol X / 1 mol XCl3 )
where x is the atomic weight of element X.
Simplifying the equation,
0.5500/ (x+380.7) = 0.2596 / (x+ 106.35)
Solving for x,
x = 138.90 g/ mol