A volume of \(x mL\) of \(5 M NaHCO _3\) solution was mixed with 10 mL of \(2 M H _2 CO _3\) solution to make an electrolytic buffer. If the same buffer was used in the following electrochemical cell to record a cell potential of 235.3 mV , then the value of \(x =\)__________ mL (nearest integer).
\(Sn ( s )\left| Sn ( OH )_6^{2-}(0.5 M )\right| HSnO _2^{-}(0.05 M ) \mid OH ^{-} \left| Bi _2 O _3(s)\right| Bi ( s )\)
Consider upto one place of decimal for intermediate calculations
\(\left[\begin{array}{ll}\text { Given : } & E _{ HSnO _2^{-} \mid Sn ( OH )_6^{2-}}^{ o }=-0.9 V \\ & E _{ Bi _2 O _3 \mid Bi }^{ o }=-0.44 V \\ & pKa _{\left( H _2 CO _3\right)}=6.11 \\ & \frac{2.303 RT }{ F }=0.059 V \\ &\operatorname{Antilog}(1.29)=19.5\end{array}\right]\)

1. A 70
2. B 75
3. C 78
4. D 80