posted by Waleed .
What is the volume of a solution of 0.5 M NaOH that must be added to adjust the pH
from 4 to 9 in 20 mL of a 100 mM solution of phosphoric acid?
Its been about 4 years since I've done this stuff. All I know is the pKa of phosphoric acid is 2.15 and I'm assuming In need to use the Henderson-Hassleback equation.
One problem I am having in helping you is that I can't figures how you started with 20 mL of 0.1M H3PO4 and it has a pH of 4. The pH of a 0.1 M H3PO4 soln is <2. The second problem is that you give a pk1 but not a pk2 (suggesting pk2 is not needed) BUT the pH of H3PO4 in which the first H is completely neutralized is sqrt(k1k2) or pH about 5 or so. If all of this is true, one must add enough base to go PAST the first equivalence point and stop somewhere just before the second equivalence point (pH about 9.8).