Metallic magnesium can be made by the electrolysis of molten MgCl 2 . (a) What m
ID: 919303 • Letter: M
Question
Metallic magnesium can be made by the electrolysis of molten MgCl2.
(a) What mass of Mg is formed by passing a current of 4.20 A through molten MgCl2 for 3.90 days? g
(b) How many minutes are needed to plate out 14.60 g Mg from molten MgCl2 using 5.50 A current?
min
A voltaic cell is constructed that uses the following reaction and operates at 298 K.
(Ered Ni+2 is -0.277)
(a) What is the emf of this cell under standard conditions?
V
(b) What is the emf of this cell when [Ni2+] = 3.07 M and [Zn2+] = 0.183 M?
V
(c) What is the emf of the cell when [Ni2+] = 0.190 M and [Zn2+] = 0.852 M?
V
Explanation / Answer
Substitute the given values in a relation ,
w = z c t ----------------------( Derived using Faraday's First Law of electrolysis )
where , w = weight of the substance deposited / liberated during electrolysis [ to be calculated in this case ] ,
-------------- c = current in amperes ( given as 4.2 amperes )
----------- , z - electrochemical equivalent weight of Magnesium* = Equivalent wt. / Faraday = 12 / 96500
*[ note that equivalent weight of magnesium is 12 ]
------------, t = time in seconds given as 3.9 days [ ie.= ( 3.9 x 24 x 60 x 60 ) ] seconds
Thus , weight of magnesium deposited = ( 12 x 4.2 x 3.9 x 24 x 60 x60 ) / 96500
----------------------------------------------------------- = 175.9874 gms
-------------------------------------------------------- or, = 0.176 Kgs
b ) Again, using the same relation [ w = zct ] , we calculate the desired time to deposit 14.60 gms of magnesium ( ie. z = 14.60 gms ) using 5.5 A current ( ie c = 5.5 amperes )
So, 14.60 = ( 12 x 5.5 x t ) / 96500
solving for t ( ie. time in seconds ) we get , t = 21346.97 seconds ,
or equal to 21346.97 / 60 = 355.78 minutes
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Hint for the other question -
a ) Write cell composition to determine anode & cathode separately,
------ use relation Eo(cell) = ER - EL & substitute given values of reduction electrode potentials for ER and EL separately to calculate desired EMF of the Cell.
b & c ) Apply Nerns't equation to calculate Electrode potential or emf of the cell under given concentrations.
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Please post the second / other question separately for details ,as a fresh question. Glad to help you.