Two beakers containing different buffer solutions of lactic acid / lactate (CH3CHOHCO2H / CH3CHOHCO2 - , or LacH / Lac- ) and quinhydrone are prepared. In one beaker the [LacH] = 0.25 M and the [Lac- ] = 0.50 M, in the second beaker the [LacH] = 0.50 M and the [Lac- ] = 0.25 M. Given that the Ka of LacH is 4.4 x10-4 , what is the voltage produced when these two cells are connected?
(i) pKa of LacH = -log (4.4 X 10^-4 )
..........................= 3.3565
(ii)
a)Calculate pH of the solutions in two beakers containing qiinhydrone separately using Handerson's equation-
pH = pKa + log ( [ Lac - ] / [ LacH ] )
b) and there from calculate the respective [H^+]
Beaker 1 ,
pH = 3.3565 + log (0.50 /.25 )
...... = 3.3565 + 0.3010
........= 3.6575
& [ H^+ ] = antilog ( - 3.6575 )
................. = 2.200 x 10^-4
similarly for second beaker ,
pH = 3.3565 + log ( 0.25 / 0.50 )
...... = 3.3565 + log (0.5 )
........= 3.3565 - 0.3010
........= 3.0555
& [ H^+ ] = antilog (-3.0555)
..................= 8.800 x 10^-4
(iii) The potential ,E Q of the quinhydrone electrode depends on the conentration of H^+ ions in solution. As worked out from Nernst equation . it is given by the relation
EQ = EoQ - { [2.303( RT / F ) ] log [ H+ ] }
or ..= EoQ + { [ 2.303 ( RT / F ) ] pH.......................................(1)
The standard reduction potential EoQ , of the quin hydrone electrode is known to be 0.6996 Thus from of (1) we have
EQ = 0.6996 + 0.0591 x pH..............................................(2)
Calculate EQ for the two beakers separately using (2)
Beaker 1 ....................EQ = 0.6996 + 0.0591 x 3.6575
.............................................= 0.9157 V
Beaker 2......................EQ = 0.6996 + 0.0591 x 3.0555
.............................................= 0.8802 V
(iv) So when the two beakers are connected a voltage produced = ( 0. 9157 - 0.8802 )
.......................................................................................................= 0.0355 V
Two beakers containing different buffer solutions of lactic acid / lactate (CH3CHOHCO2H / CH3CHOHCO2 - ,...
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