A solution containing 0.319 g of CrCl_​3.6H₂O was passed through a cation exchange resin and the acid coming out of the resin required 28.5 mL of 0.125  M NaOH. Determine the correct structural formula of the complex ? 

1. Number of moles of CrCl₃.6H₂O (n₁) = $\frac{Given mass}{Molar Mass}=\frac{0.319g}{266g/mole}=0.001moles$

2. Number of moles of NaOH (n₂) = $$\begin{gathered} Molarity \times volume of solution in L \\ = 0.125 mole/L \times 28.5 \times {10}^{-3} L \\ = 0.003 moles \end{gathered}$$

3. $\frac{n_1}{n_2}=\frac{0.001}{0.003}=\frac{1}{3}$

4. Thus,3 moles of NaOH reacts with 1 mole of complex. So,1 mole of complex has 3 Cl^- ions to forms 3NaCl  with 3 moles of NaOH. Therefore,the structure is : [Cr(H₂O)₆]Cl₃ and reaction is :
 
          
             [Cr(H₂O)₆]Cl₃ + 3NaOH $\to$  [Cr(H₂O)₆](OH)₃ + 3NaCl