Use the Henderson-Hasselbalch equation.
pH = pKa + log [(base)/(acid)]
H^+ + HPO4^-2 ==> H2PO4^-
You start with 0 millimoles H^+.
You start with 5 x 1 = 5 millimoles HPO4^-2
You start with 5 x 1 = 5 millimoles H2PO4^-1.
Now you add 4 x 1 = 4 millimoles H^+.
That reacts with 4 millimiles HPO4^-2 to form 4 millimoles H2PO4^- (plus the 5 already there = 9 millimoles H2PO4^-).
That leaves 5 millimoles HPO4^-2 - 4 millimoles that reacted with H^+ = 1 millimole HPO4^-.
Plug into HH equation and solve for pH.
Check my work. It's easy to leave type a wrong charge, etc.
What happens when 4ml of 1M HCL is added to 100ml buffer made using 5ml 1M Na2HPO4 and 5ml 1M NaH2PO4? The pKa for H+ + HPO4- = H2PO4 is 6.82.
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