Answer
The unknown element is $Ti$
Work Step by Step
We know from observing the formula of this compound that it contains 4 atoms of Chlorine $Cl$ and one atom of an unknown substance. From the periodic table, we know that the molar mass of Chlorine is $35.45 g/mol$
Multiply the atomic mass of $Cl$ by the four atoms in the compound and we get $141.81 g/mol$ as the molar mass of the total chlorine in the compound. Since we know Chlorine in its entirety makes up 75% of the compound's mass, we can divide the molar mass of chlorine in the substance by percent mass $141.81 g/mol \div 0.75 = 189 g/mol$, which is the total mass of the compound. Now we take the total percent mass (100%) and subtract it by the percent mass of $Cl$ to get the percent mass of the unknown element $100 - 75 = 25$. Finally, we can multiply the percent mass of the unknown element by the total mass of the compound $189 g/mol \times 0.25 mass = 47 g/mol$. The final result is the molar mass of the unknown element, in which if we search on the periodic table, appears to be the molar mass of Titanium $Ti$. And so, the compound is $TiCl_{4}$.
