Calculate the change in pH when 5.00 mL of 0.100 M HCl(aq) is added to 100.0 mL of a buffer solution that is 0.100 M in NH3(aq) and 0.100 M in NH4Cl(aq).

1 answer

The pH at the beginning is
pH = pKa + log(base)/(acid)
Substitute pKa, base, and acid, and solve for initial pH.

You have 100 mL 0.100M NH3 and 0.100M NH4Cl = 10 millimoles of each. You're adding 5.00 mL of 0.100M HCl = 0.500 millimols.

.......NH3 + H^+ ==> NH4^+ + H2O
I......10.....0.......10........
add...........0.5...............
C......-0.5...-0.5.....+0.5
E......9.5.....0.......10.5

Now substitute the E line into the HH equation and solve for pH, then take the difference between the two pH values to arrive at the difference.