FeCl3 reacts with NaOH follows the formula: FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl. In this reaction, the molar ratio of the two reactants is 1:3. So in order for both reactants to react with each other completely, ratio needs to be 1:3. Any other ratio is not possible to give a complete reaction, in another word, there would be one or the other reactant left over.