The element X is titanium.
According to the question, the percentage by mass of Cl in XCl4 is 75.0%.
The percentage by mass is obtained from;
Mass of element in the compound/molar mass of compound * 100
Hence;
Let the mass of the unknown element be X
75 = 142/X + 142 * 100
75 (X + 142) = 142 * 100
75X + 10650 = 14200
75X = 14200 - 10650
X = 14200 - 10650/75
X = 47
Hence, X is titanium.
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