Nitrogen in wastewater is in the form of ammonia NH 3 (aq) and ammonium ion NH 4

Question

Nitrogen in wastewater is in the form of ammonia NH3(aq) and ammonium ion NH4+(aq). Consider the following equilibrium reaction and mass balance reaction:
NH4+(aq) <------------> NH3+(aq) + H+(aq)


Total concentration is given:


NT = [NH4+ (aq)] + [NH3 (aq)] = 8.1 x 10-4 M (mol/L)

Combining these two equations yields:

[NH4+(aq)] = [H+]NT / ([H+]+Ka)

[NH3(aq)] = KaNT / ([H+]+Ka)

Analysis of an aqueous solution indicates that the pH is equal to 7.0
a) Calculate [NH4+(aq)] and [NH3(aq)]
b) At what pH would the concentrations [NH4+(aq)] and [NH3(aq)] be equal?

Explanation / Answer

A)                     NH4+ <-------> NH3 + H+

at t=0               x                  0         0

at equlibrium     x(1-y)          x*y         x*y

but given    x(1-y) + xy = 8.1*10^-4

               NH4+ = x = 8.1*10^-4

also pH = 7

-log H+ = 7

H+ = 1*10^-7

thus NH3 = xy = 1*10^-7

B) NH3 = NH4+

KaNT / ([H+]+Ka) = [H+]NT / ([H+]+Ka)

Ka = H+

pH = 1/2 * (pKw + pKa + logc)

pH = 1/2( 14 + pH + log(8.1*10^-4) )

2pH = 14 + pH -3.0

pH = 10.91

Related Questions

Navigate

• Browse (All)
• Browse N
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.