Problem: Calculate the pH of a solution formed by mixing 250.0 mL of 0.15 M NH4CI with 100.0 mL of  0.20 M NH3. The Kb for NH3 is 1.8 x 10-5. A) 8.98 B) 9.25 C) 9.53 D) 9.13 E) 4.74

FREE Expert Solution

Henderson-Hasselbalch equation.

pH=pKa+log (conjugate baseweak acid)


Determine the composition of the solution.

NH4Cl → breaks up in the solution

NH4Cl → NH4+ + Cl-

Equilibrium reaction:  NH4+ + H2O ⇌ NH3 + H3O+

NH4+ → weak acid

NH3 → conjugate base

• The solution is composed of a weak acid and its conjugate base, therefore we have a buffer.


Calculate the pKa of the NH4+.

KwKb=Ka·KbKbKa=KwKbKa=1.0×10-141.8×10-5

Ka = 5.555x10-10


pKa=-logKapKa=-log (5.555×10-10)

pKa = 9.25



Determine the initial moles of the weak acid and conjugate base present.

Recall molarity is:

Molarity (M)=molL

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Problem Details

Calculate the pH of a solution formed by mixing 250.0 mL of 0.15 M NH4CI with 100.0 mL of  0.20 M NH3. The Kb for NH3 is 1.8 x 10-5

A) 8.98 

B) 9.25 

C) 9.53 

D) 9.13 

E) 4.74

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