Question

What would be the magnitude of the EMF of the following cell:

At $25^{\circ} C$ ? The ionization constant of acetic acid, $K_{ a } \sim 10^{-5}$, while that of formic acid, $K_{ a } \sim 10^{-4}$

• A. $0.0295\, V$
• B. $0.059\, V$
• C. $-0.059$
• D. $-0.0295 \, V$

Answer 1

Answer: (B)

At anode:
$0.1\, M$ acetic acid $+0.1\, M$ sodium acetate $\Rightarrow$ Acidic buffer
$\therefore pH _{ a }= p K_{ a }+\log \left[\frac{\text { Salt }}{\text { Acid }}\right] \left( p K_{ a \left( CH _3 COOH \right)}=5\right) $
$=5+\log \frac{0.1}{0.1}=5$
At cathode:
$0.1 \, M$ formic acid $+0.1 \, M$ sodium formate $\Rightarrow$
Acidic buffer
$\therefore pH _{ c }= p K_{ a }+\log \left(\frac{0.1}{0.1}\right) \left[ p K_{ a ( HCOOH )}=4\right] $
$ pH _{ c }=4 $
$E_{\text {cell }}=-0.059\left( pH _{ c }- pH _{ a }\right)=-0.059(4-5) $
$=0.059\, V$