Question

Calculate the $pH$ of a buffer solution prepared by dissolving $10.6g$ of $N{a}_{2}C{O}_3$ in $500ml$ of an aqueous solution containing $80ml$ of $1MHCl.K_a$ for $HC{O}_3^{-}=6\times 10^{-11}$

• A. 8.6
• B. 9.6
• C. 11.6
• D. 12.6

Answer 1

Answer: (B)

$N{a}_{2}C{O}_3+HCl\to NaCl+NaHC{O}_3$

$M_{\text{eq. }}$ before reaction $\frac{10.6}{106}\times 100080\times 1$



The solution has $N{a}_{2}C{O}_3$ and $HC{O}_3^{-}$ and thus acts as buffer

$pH=-\log K_a+\log \frac{\left[C{O}_3^{-2}\right]}{\left[HC{O}_3^{-}\right]}$

$=-\log 6\times 10^{-11}+\log \frac{20}{80}=9.6$