A Hittorf cell fitted with Ag/AgCl electrodes is filled with 0.0106 molal aqueous HCl (molar mass: 36.458). A 2.00 mA current was passed for 10800 sec. The cathode solution was then found to weigh 51.7436 g and contain 0.0267 g HCl after analysis. What is the transport number of H+?

a. 0.177

b. 0355

c. 0.645

d. 0.823

Answer :

Explanation:

The given data is as follows.

            Current = [tex]2 \times 10^{-3}[/tex] A

            Time = 10800 sec

Weight of cathode solution = 51.7436 g

Weight of HCl after analysis = 0.0267 g (in cathode solution)

Weight of anode solution = 52.0461 g

Weight of HCl in anode solution = 0.0133 g

Amount of electrolyte present = molality × molar mass

                                                   = [tex]0.0106 \times 36.4 \times 10^{-3}[/tex]

                                                   = [tex]0.385 \times 10^{-3} g[/tex]

Calculate the amount of HCl initially present as follows.

          Electricity passed = [tex]2 \times 10^{-3} \times 10800[/tex] = 21.6 C

Hence, amount of electrons passed will be as follows.

                     [tex]\frac{21.6 C}{96500 C/mol}[/tex]

                    = 0.00024 mol

Therefore, mass of HCl initially present is as follows.

                     = (51.7436 - 0.0267)      

                     = 51.7169 g of water

               = [tex]\frac{0.3856 \times 10^{-3}}{1g H_{2}O} \times 51.7169 g H_{2}O[/tex]    

               = 0.01994 g

Mass of HCl present after the electrolysis in 51.7169 g of [tex]H_{2}O[/tex] = 0.0267 g

           Mass of HCl gained = 0.0267 - 0.01994

                                             = 0.00676 g

Hence, the amount of HCl gained = [tex]\frac{0.00676 g}{36.5 g/mol}[/tex]    

                                                        = 0.0001853 mol

Therefore, gain of [tex]Cl^{-}[/tex] in cathodic compartment = 0.000224 mol

So, transport number of [tex]H^{+}[/tex] = [tex]\frac{\text{Amount of HCl gained}}{\text{moles of chlorine ions gained}}[/tex]

                                                = [tex]\frac{0.000185}{0.00024}[/tex]

                                                = 0.823

Thus, we can conclude that the transport number of [tex]H^{+}[/tex] is 0.823.

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