A buffer, consisting of H2PO4− and HPO42−, helps control the pH of physiological fluids. Many carbonated soft?
A buffer, consisting of H2PO4− and HPO42−, helps control the pH of physiological fluids. Many carbonated soft drinks also use this buffer system. What is the pH of a soft drink in which the major buffer ingredients are 5.20 g of NaH2PO4 and 6.20 g of Na2HPO4 per 355 mL of solution?
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First determine the concentrations of each component:
(5.20 g NaH2PO4) x [(1 mole NaH2PO4)/(119.98 g NaH2PO4)] x [1/(0.355 L)] = 0.122 M NaH2PO4
(6.20 g Na2HPO4) x [(1 mole Na2HPO4)/(141.96 g Na2HPO4)] x [1/(0.355 L)] = 0.123 M Na2HPO4
Look up the pKa for this component (it will correspond to Ka2 of phosphoric acid) and use the Henderson-Hasselbach equation:
pH = pKa + log([NaH2PO4]/[Na2HPO4])