Answer
Atomic mass of X = 51.98
Therefore X is Chromium [Cr]
Work Step by Step
Percentage of an element in a compound =(Total weight of the element in the compound$\times$100)$\div$ Molecular mass of that Compound.
The molecular mass of the compound =(Total weight of the element in the compound$\times$100)$\div$ Percentage of the element.
In $XCl_3$ ,
Total weight of Cl = $(3\times35.5) = 106.5$
Percentage of Cl = 67.2
Then, molecular mass of $XCl_3 = (106.5 \times100)\div 67.2 = 158.48$
Therefore mass of X in $XCl_3 = (158.48-106.5) = 51.98$
Since there is only one X in $XCl_3$ ,
Atomic mass of X = 51.98
The element with atomic mass 51.98 is Chromium [Cr].
