$3.$ Our mordant will be aluminum ion $($derived from alum$).$

$($a$)$ The formula of alum is unusual; it is a salt with two cations. Show with clear charges that the formula is reasonable based on what you know about the ions it contains.

$($b$)$ The structure of PETE is shown below. Consider the polarities and $($:${}^{-}\}$of the bonds shown. Add to this sketch two aluminum ions, showing the kinds of ion$-$dipole attractions one would expect for this mordant and this fabric. Label these interactions with dotted lines.