(a) Which of two of the following compounds would you mix to make a buffer of pH 7.45: H3PO4 (FM: 98.00), NaH2PO4 (FM: 119.98), Na2HPO4 (FM: 141.96), and Na3PO4 (FM: 163.94)?
(b) If you wanted to prepare 1.00 L of buffer with a total Phosphate concentration of 0.050 M, how many grams of each of the two compounds would you mix?
(c) If you did what you calculated in part (b) you would not get a pH of exactly 7.45. Explain how you would really prepare this buffer in the lab.
Answer:
(a) NaH2PO4 and Na2HPO4 would be simplest.
(b) 4.55 g Na2HPO4 + 2.15 g NaH2PO4
(c) Weigh out 0.0500 mol Na2HPO4 and dissolve in 900 mL water Add HCl until pH is 7.45, and dilute with water.