The answer to yo question is ( it has three OH groups).
The branched structure isomer will require less energy to melt than the straight chain isomer
explanation
Branched structure isomer has weak intermolecular forces of attraction as compared to straight chain isomers. In addition the branched isomer has a low boiling point as compared to straight chain isomers. Since boiling require the of the intermolecular forces tend to have lower boiling point than straight chain
The variable is what changes during an experiment. Hopefully this helped! :)
You got the questions correct
The charge for this compound is positive. For Fe, it's charge is positive 3, and for OH, it's charge is negative 1. You would then criss cross the charges of each and come out with Fe(OH)3. I hope this helped!! :)