Redox Reactions and Electrochemistry: JEE Main 2026 Chemistry question

A volume of x mL of 5 M $\mathrm{NaHCO_3}$ solution was mixed with 10 mL of 2 M $\mathrm{H_2CO_3}$ 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). $\mathrm{Sn(s)\,|\,Sn(OH)_6^{2-}\ (0.5\ M)\,|\,HSnO_2^-\ (0.05\ M)\,|\,OH^-\,|\,Bi_2O_3(s)\,|\,Bi(s)}$. Consider upto one place of decimal for intermediate calculations. [Given : $E^\circ_{\mathrm{HSnO_2^-|Sn(OH)_6^{2-}}} = -0.9$ V, $E^\circ_{\mathrm{Bi_2O_3|Bi}} = -0.44$ V, $\mathrm{p}K_a(\mathrm{H_2CO_3}) = 6.11$, $\frac{2.303RT}{F} = 0.059$ V, Antilog (1.29) = 19.5]