Chemistry Question from JEE Main 2026 · 28 January, Shift 2

A volume of $x$ mL of $5 \, \text{M}$ NaHCO$_3$ solution was mixed with $10$ mL of $2 \, \text{M}$ 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 $235.3 \, \text{mV}$, then the value of $x =$ _____ mL (nearest integer).

$$\ce{Sn(s) | Sn(OH)6^{2-} (0.5 M) | HSnO2^- (0.05 M) | OH^- | Bi2O3(s) | Bi(s)}$$

Given:

$$E^\circ(\ce{HSnO2^- / Sn(OH)6^{2-}}) = -0.90 \,V, \quad E^\circ(\ce{Bi2O3 / Bi}) = -0.44 \,V$$ $$pK_a(\ce{H2CO3}) = 6.11, \quad \frac{2.303RT}{F} = 0.059 \,V, \quad \text{Antilog}(1.29) = 19.5$$