Question

What is the pH of a solution that results form mixing together equal volumes of a 0.0700 M solution of ammonia and 0.0350 M solution of hydrochloric acid? (pKa of NH4 = 9.25)

Answer 1

pH = 9.25

Explanation

After mixing the solution

concentration of NH3 = 0.0700M/2 = 0.0350M

concentration of HCl = 0.0350M/2 = 0.0175M

H⁺ + NH3 --------> NH4⁺

1:1 molar reaction

0.0175M of HCl react with 0.0175M of NH3 to give 0.0175M of NH4⁺

remaining concentration of NH3 = 0.0350M - 0.0175M = 0.0175M

concentration of NH4⁺ formed = 0.0175M

Henderson - Hasselblch equation is

pH = pKa + log([A^-]/[HA])

pH = 9.25 + log(0.0175M/0.0175M)

pH = 9.25