Can someone please help me step by step with this question I

Can someone please help me step by step with this question? I have been attempting it for hours and no luck.

Using a 0.25 M phosphate buffer with a pH of 6.1, you add 0.77 mL of 0.55 M HCI to 59 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.) Using a 0.25 M phosphate buffer with a pH of 6.1, you add 0.77 mL of 0.55 M NaOH to 59 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)

Solution

pH of phosphate buffer = pka2 + log(HPO4^-2/H2PO4^-)

   pka2 phosphate buffer = 6.8

total no of mol of buffer = 0.25*59 = 14.75 mmol

H2PO4^- + HPO4^-2 = 14.75

6.1 = 6.8+log(x/(14.75-x))

no of mol of HPO4^-2 = x = 2.45 mmol

no of mol of H2PO4^- = 14.75-2.45 = 12.3 mmol

after addtion of HCL

no of mol of hcL added = 0.77*0.55 = 0.4235 mmol

pH of phosphate buffer = pka2 + log(HPO4^-2 - HCl/H2PO4^- + HCl)

= 6.8+log((2.45-0.4235)/(12.3+0.4235))

pH = 6.00

after addition of NaOH

no of mol of naoh added = 0.77*0.55 = 0.4235 mmol

pH of phosphate buffer = pka2 + log(HPO4^-2 + NaOH/H2PO4^- - NaOH)

               = 6.8+log((2.45+0.4235)/(12.3-0.4235))

pH = 6.18