Question

A compound, ${\mathbf{XF}}_{\mathbf{5}}$, is 42.81% fluorine by mass. Identify the element X. What is the molecular structure of ${\mathbf{XF}}_{\mathbf{5}}$?

Short answer

The element X is Iodine

Molecular structure of$X{F}_5$

Step-by-step solution

 Calculate the number of moles:

Number of moles $\mathrm{F}=42.81\mathrm{g}\mathrm{F}\times \frac{1molF}{19.00\mathrm{g}\mathrm{F}}=2.253mol\mathrm{F}$

Number of moles of$\mathrm{X}\mathrm{in}{\mathrm{XF}}_5=2.253\mathrm{mol}\mathrm{F}\times \frac{1\mathrm{mol}\mathrm{X}}{5\mathrm{mol}\mathrm{F}}=0.4506\mathrm{mol}\mathrm{X}$

Mass of $\mathrm{X}=100-42.81=57.19\mathrm{g}$

So the molar mass of $\mathrm{X}=\frac{57.19\mathrm{g}\mathrm{X}}{0.4506\mathrm{mol}\mathrm{X}}=126.9\mathrm{g}/\mathrm{mol}$

This is the molar mass of element Iodine.$\left(I\right)$

Show the molecular structure:

$$\begin{gathered} {\mathrm{If}}_5=7+5\left(1\right)=7+5=12\mathrm{electrons} \\ =6\mathrm{pair}\mathrm{of}\mathrm{electrons} \\ =5\mathrm{bond}\mathrm{pair}+1\mathrm{Ione}\mathrm{pair} \end{gathered}$$

The geometry of the compound is octahedral. But due to presence of one lone pair the structure of the compound is square pyramidal.

Square pyramidal structure