Question

An aqueous solution 10 g of an optically pure substance diluted to 500ml with water and placed in a polarimeter tube 20 cm long. The measured rotation was -6.16. Calculate the specific rotation of this compound .

Answer 1

Explanation:

Formula to calculate specific rotation is as follows.

        Specific rotation ($[\alpha]$) = $\frac{\alpha}{c} \times l$

where,    $\alpha$ = observed rotation

                   c = concentration in g/ml

                   l = path length in dm

It is given that,

             $\alpha = -6.16^{o}$    

               c = $\frac{10 g}{500 ml}$ = 0.02 g/ml

              l = 20 cm = 2 dm   (as 1 dm = 10 cm)

Therefore, calculate the specific rotation as follows.

      Specific rotation ($[\alpha]$) = $\frac{\alpha}{c} \times l$

                        = $\frac{-6.16^{o}}{0.02 g/ml} \times 2 dm$

                        = $-616^{o}$

Thus, we can conclude that the specific rotation of this compound is $-616^{o}$.