Calculate how many moles of h3o+ can be neutralized

Question

Consider a buffer solution that contains 0.50 MNaH₂PO₄ and 0.20 MNa₂HPO₄. 

a. Calculate the pH of this buffer solution.

b. Calculate the pH after 0.120 g ofsolid NaOH is added to 150 mL of this buffer solution.c. If the acceptable buffer range of the buffer solution is ±0.10 pH units, calculate how many moles of H₃O⁺ can be neutralized by 250 mL of the buffer.