(Please show all steps and include explanations):
A buffer solution contains 0.0237 mol sodium dihydrogen phosphate and 0.0165 mol of sodium hydrogen phosphate in a solution volume of 800. mL. What is the pH after 0.0022 mol HNO3 is added to the solution?
A) 6.74
B) 6.81
C) 6.94
D) 7.07
E) 7.10
Apparently, the answer is C, 6.94, but I have absolutely no idea as to why.