Question

The groundwater sample was analysed. The measured pH of the test water at 25°C had a value, pH = 3.8. The concentration of Zn^2+ ions in water was also determined to be 4.132·10^-2 mol/L.

Groundwater is in equilibrium with sediment containing MnO2 and ammonia. Calculate the

concentration of NH4^+ ions under these conditions.

Standard reduction electrode potentials for reactions:

Zn ^2+ + 2e →Zn E°= -0.76V

 2MnO 2 + 2NH4^+ +2e → Mn2O3 +2NH3

+ H2O

 E°= 0.74 V

Write a sum cell reaction.

Answer 1

concentration of NH4+ ions = $4.132× 10^{-2}$Mol/L

$Zn ^{2+}+ 2e →Zn$ $E°= -0.76V$

$2MnO_2 + 2NH_4+ 2e → Mn_2O_3 +2NH_3+ H_2O$

$E°= 0.74 V$

Sum cell reaction = $0.74 -0.76 =-0.02V$