To find the pH of a buffer composed of H2PO4- (aq) and HPO42- (aq), which pKa value would you use in Henderson-Hasselbalch equatio? : pKa2 = 7.21
pH = 7.43
Explanation
mass KH2PO4 = 24.0 g
moles H2PO4- = (mass KH2PO4) / (molar mass KH2PO4)
moles H2PO4- = (24.0 g) / (136.0855 g/mol)
moles H2PO4- = 0.17636 mol
concentration H2PO4- = (moles H2PO4-) / (volume of solution in Liter)
concentration H2PO4- = (0.17636 mol) / (1.0 L)
concentration H2PO4- = 0.17636 M
mass Na2HPO4 = 41.0 g
moles Na2HPO4 = (mass Na2HPO4) / (molar mass Na2HPO4)
moles Na2HPO4 = (41.0 g) / (141.96 g/mol)
moles Na2HPO4 = 0.2888162 mol
concentration HPO42- = (moles Na2HPO4) / (volume of solution in Liter)
concentration HPO42- = (0.2888162 mol) / (1.0 L)
concentration HPO42- = 0.2888162 M
According to Henderson - Hasselbalch equation,
pH = pKa + log([conjugate base] / [weak acid])
pH = pKa2 + log([HPO42-] / [H2PO4-])
pH = 7.21 + log(0.2888162 M / 0.17636 M)
pH = 7.21 + log(1.66)
pH = 7.21 + 0.22
pH = 7.43
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