Question

Calculate the $pH$ of a buffer solution prepared by dissolving $10.6 \,g$ of $Na _{2} CO _{3}$ in $500 \,ml$ of an aqueous solution containing $80\, ml$ of $1 \,M \,HCl . K _{ a }$ for $HCO _{3}^{-}=6 \times 10^{-11}$

• A. 8.6
• B. 9.6
• C. 11.6
• D. 12.6

Answer 1

Answer: (B)

$Na _{2} CO _{3}+ HCl \rightarrow NaCl + NaHCO _{3}$

$M_{\text {eq. }}$ before reaction $\frac{10.6}{106} \times 1000 \quad 80 \times 1$



The solution has $Na _{2} CO _{3}$ and $HCO _{3}^{-}$ and thus acts as buffer

$ pH =-\log K_{ a }+\log \frac{\left[ CO _{3}^{-2}\right]}{\left[ HCO _{3}^{-}\right]}$

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