Sodium dihydrogen phosphate NaH_2PO_4 (monobasic) and sodium hydrogen phosphate

Question

Sodium dihydrogen phosphate NaH_2PO_4 (monobasic) and sodium hydrogen phosphate Na_2HPO_4 dibasic) are a weak acid and its conjugate base pair that are mixed to make a buffer with pH uc !S'C) ind. The pKa of the acid-base transition below is 7.2 Use the Henderson-Hasselbalch equation below: pH = pKa + log [A-]/[HA] Determine the molar ratio of NaH_2PO_4 and Na_2HPO_4 in the buffer with pH = 7.2 What would be the concentrations of NaH_2PO_4and Na_2HPO_4 in 20 mM buffer with pH = 7.2? Calculate the amount of monobasic and dibasic solids to prepare 20 ml of a 20 mM buffer, pH = 7.2 (use the molecular weights on the NaH_2PO_4and Na_2HPO_4to determine how much to weigh out).

Explanation / Answer

5. Buffer

a. with pH = pKa = 7.2

[NaH2PO4] = [Na2HPO4]

So we have equal moles of both species in solution

i. In a 20 mM buffer solution

10 mM NaH2PO4

10 mM Na2HPO4

b. to prepare a 20 ml of 20 mM buffer solution

NaH2PO4 = 10 mM x 20 ml x 120 = 24.0 g

Na2HPO4 = 10 mM x 20 ml x 142 = 28.4 g

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