Finding pH of an Amphoteric Salt - Example 6E.1 in the textbook

[Shimona_Gupta_3I]

I have been stuck on/confused about Example 6E.1 in the textbook:

Salts are often used to create solutions with specific pH values. Suppose you need to prepare a salt solution with a pH of about 4.5 and have available sodium dihydrogen phosphate, NaH2PO4, and sodium citrate, Na2HC6H5O7. Which should you use? Estimate the pH of (a) 0.20M NaH2PO4; (b) 0.20M Na2HC6H5O7. For citric acid, pKa2 = 5.95 and pKa3 = 6.39.

The textbook provides a solution, including mentioning something about S >> Kw/Ka2 and S>>Ka1, however I do not completely understand where and how to start this problem.

[Alba Mendis 1A]

Polyprotic acids can donate more than one proton, so Ka1 refers to the first dissociation of proton, and Ka2 the second; the S is the initial concentration of salts. As long as S is a lot bigger than Ka1 and Kw/Ka2, you can use the equation provided by the textbook: pH = 1/2 (pKa1 + pKa2). You can then use Table 1 to find out the Ka values and derive the appropriate pKa values. Hopefully this helped!