Question

A buffer solution is composed of 1.212 g of KH2PO4 and 5.740 g of Na2HPO4. (a) What is the pH of the buffer solution? (b) What mass of KH2PO4 must be added to decrease the buffer solution pH by 0.1 unit from the value calculated in part (a)?

Answer 1

pka of KH2PO4/Na2HPO4 buffer = 7.2

pH = pka + log [Na2HPO4/KH2PO4]

moles of KH2PO4 = 1.212gm /136.09 g/mol =0.0089 moles

moles of Na2HPO4 = 5.74g/141.96g/mol = 0.04 moles

pH = 7.2 + log [0.04/0.0089]= 7.85

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7.75 = 7.2 + log [Na2HPO4/KH2PO4]

7.75 = 7.2 + log[0.04/KH2PO4]

or, [KH2PO4] = 0.011 moles

mass of KH2PO4 = 0.011 moles * 136.09gm/mol = 1.534 gm

mass of KH2PO4 tht has to be added = 1.534-1.212 =0.322 gm