In: Chemistry
Using a 0.25 M phosphate buffer with a pH of 6.0, you add 0.79 mL of 0.45 M NaOH to 59 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)
Since the pH = 6
It is nearer to second dissociation pKa value
pKa2 = 7.21
H2PO4 - ---------------> HPO4-2
pH = pKa2 + log [ HPO4-2] / [H2PO4-]
6 = 7.21 + log [HPO4-2] / [H2PO4-]
[HPO4-2] / [H2PO4-] = 0.061
Moles of HPO4-2 + Moles of H2PO4- = 0.25*59/1000 = 0.01475
Moles = molarity*V in L
HPO4-2 = 0.061*H2PO4-
0.061*H2PO4- + H2PO4- = 0.01475
Moles of H2PO4 - = 0.0139
MOles of HPO4-2 = 0.000848
MOles of NaOH = 0.45*0.79/1000 = 0.0003555
H2PO4- + NaOH --------> HPO4-2
0.0139 0.0003555 0.000848
0.0139-0.0003555 0 0.000848+0.0003555
pH = 7.21 + log [0.000848+0.0003555] / [0.0139-0.0003555]
pH = 6.158