The charge on $\mathrm{Fe}(\mathrm{OH}{)}_3$ sol is due to 

${\mathrm{FeCl}}_3$ is added to excess hot water, a positively charged sol of ferric hydroxide is formed due to adsorption of ${\mathrm{Fe}}^{3+}$ ions. 

$\mathrm{Fe}(\mathrm{OH}{)}_3/{\mathrm{Fe}}^{3+}$

Positively charged