chemistry
posted by matt .
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.

Use the HendersonHasselbalch 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.