Greetings everyone,

I am currently struggling trying to understand buffer systems and more generally speaking, neutralising titrations of all varieties. Specifically, I came across this problem;

1.) Determine the pH of buffer solutions obtained by mixing 50,00 mL of 0,200M NaH

_{2}PO

_{4} with;

a) 50,00 mL of 0,120M HCl (1,97)

b) 50,00 mL of 1,120M NaOH (7,37)

K

_{1}=7,11*10

^{-3}K

_{2}=6,34 *10

^{-8}K

_{3}=4,2*10

^{-13}for a), my idea was to write the following equation:

NaH

_{2}PO

_{4} + HCl

H

_{3}PO

_{4} + NaCl

which to my knowledge, can be regarded as a one-way?

Next, I wrote:

H

_{3}PO

_{4} + H

_{2}O

H

_{2}PO

_{4}^{-} + H

_{3}O

^{+}So, NaH

_{2}PO

_{4} and HCl react at a molar ratio of 1:1, which is 0,006 mol (some sodium dihydrogenphosphate is left out), resulting in a 0,06M solution of H

_{3}PO

_{4} (total volume is now 100 mL)

Using the K

_{1}, I figured out that in the second equation, [H

_{3}O

^{+}] equals 0,017403, which in turn, results in the pH of 1,76. Did I go wrong somewhere or are the solutions wrong (written above)? Also, I'm guessing b) has pretty much the same idea? I got 6,68 for b) but I think this is wrong, since I have a strong feeling the system would have a basic pH.

Thanks!