What is the ph of a solution containing 0.12mol/l of nh4cl and 0.03 mol/l of naoh (pka is 9.25) ?

Given the following:

Concentration of residual (NH4) + = 0.12 - 0.03 = 0.09 mol / L

Concentration of created NH3 = 0.03 mol / L

The (NH4) + reacts with all (OH) - to form NH3 and H2O (Water)

The resultant solution is NH4 + / NH3 buffer system in which (NH4) + is acid and the NH3 is base or as called the salt.

For a buffer system:

pH = pKa + Log (acid) / (salt)

= 9.25 + Log [ (NH4) + ] / [ NH3 ]

= 9.25 + Log [ 0.09 ] / [ 0.03 ]

= 9.25 + Log 3

= 9.25 + 0.4771

pH = 9.7271 or 9.73 which is rounded up to 2 decimal places