Question

A buffer system contains 0.30 M NH4+ and 0.15 M NH3. pKa of NH4+ is 9.25. How many moles of NaOH must be added to 1.00 L of this solution to increase the pH to 9.25? Hint: What is the (NH3) / (NH4+) when the pH=pKa?

Answer 1

If you want pH = pKa then you want (NH3) = (NH4^+). You get that from
pH = pKa + log (NH3)/(NH4^+)
9.25 = 9.25 + (NH3)/(NH4^+)
and that means (NH3) = (NH4^+)
......NH4^+ + OH^- ==> NH3 + H2O
I.....0.3..............0.15
add............x.............
C......-x.....-x.......+x
E...0.3-x......0......0.15+x

So you want 0.3-x = 0.15+x
Solve for x (in mols/L) and convert to mols in the 1 L.