Question

Chloride ions, Cl- (aq) can be electrolyzed in water to Cl2 (aq). If 198.3 mA of current flows through a platinum electrode immersed in a 0.200 M Br solution for 4.44 hr, how many grams of cl2 will be produced? MM Cl2= 35.45 g.


I got 1.15 grams but im not sure if i am doing this right

Answer 1

1 - As per question certain amount of electricity is passed through a Bromide solution, hence Br would be the product of electrolysis and not Cl. However indirectly the question also refers to electrolysis of chloride ion in aqueous solution liberating Cl₂ (aq ). Hence , we should consider that there is a simultaneous electrolysis of the chloride solution ,and the same amount of electricity is passed through water to yield Cl₂ .  

2 - The question ,therefore,calls for the application of Faraday's Second law of electrolysis which is stated as-

' If the same quantity of electricity is passed through different electrolytes the weights of different substances deposited is proportional to their chemical equivalent weights respectively'.

3 - Let us calculate the gram equivalent of Bromide liberated by passing 198.3 mA current for 4.44 hours through 0.200M Br solution;

Since 96,500 coulombs deposit one gram equivalent weight of any substance in this case Br ie. = 80.00 gms {198.3 x 4.44 x 60 x 60 ) /1000 } Coulombs would deposit = { 80.00 ( 198.3 x 4.44 x60 x 60 ) } /(1000 x 96500 )

   = 2.6276 gms.

or, = 2.6276 / 80

= 0.03284 gm. equivalent weight of Br

Hence , Cl deposited by passing the same quantity of electricity would also be = 0.03284 gm. equivalent weight

or, = 0.03284 x 35.48

= 1.16 gms.

Note - Your answer, therrefore , seems to be correct , but it should be desirable to check the steps and procedure being followed.

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note that -

Amount of electricity = (198.3 /1000 ) ( 4.44 x 60 x60 ) Coulomb [ ie.Current in Amperes X time in seconds ]

One Faraday = 96,500 coulomb

Equivalent wt . of Br = 80.00