Question

If the pka of hcho2 is 3.74 and the ph of an hcho2/nacho2 solution is 3.89, it means that

Answer 1

Hi, the question has some missing part. The complete question should be " If the $pK_{a}$ of $HCHO_{2}$ is 3.74 and the pH of an $HCHO_{2}/NaCHO_{2}$ solution is 3.89, it means that -

(a) $[HCHO_{2}]=[NaCHO_{2}]$

(b) $[HCHO_{2}]> [NaCHO_{2}]$

(c) $[HCHO_{2}]< [NaCHO_{2}]$

Which of the options are true?"

Answer: Option (c) is true.

Explanation:

$HCHO_{2}/NaCHO_{2}$ is an weak acid-conjugate base buffer. Hence, in accordance with Henderson-Hasselbalch equation-

                  $pH=pK_{a}(HCHO_{2})+log(\frac{[CHO_{2}^{-}]}{[HCHO_{2}]})$

Here, pH = 3.89, $pK_{a}$ of $HCHO_{2}$ = 3.74

So, $3.89=3.74+log(\frac{[CHO_{2}^{-}]}{[HCHO_{2}]})$

or, $\frac{[CHO_{2}^{-}]}{[HCHO_{2}]}$ = 1.41 (>1)

So,  $[CHO_{2}^{-}]> [HCHO_{2}]$

or, $[NaCHO_{2}]> [HCHO_{2}]$

Hence, option (c) is correct.