Solution: Calculating pH of a Solution with NH4+Cl and NaOH

[kasse]

[SOLVED] pH of buffer solution

Homework Statement

Calculate the pH of a solution made by mixing 750 mL 1.00 M NH₄⁺Cl and 250 mL 1.00 M NaOH

The Attempt at a Solution

NH₄⁺ will react with H₂O to form NH₃ and H₂O⁺. If my calculations are correct, [H⁺] after the reaction will be 2.07E-5.

Then H⁺ will react with OH^-. But since there's almost no H⁺ compared to NO^-, this reaction is irrelevant. The pOH is therefore

pOH = -log [0.25], and the pH is 13.4

which is wrong. What's my mistake(s)?

 

[Borek]

You should use Henderson-Hasselbalch equation. Your approach neglects shifting of the NH₄⁺ dissociation equilibrium to the right with H⁺ neutralization.