I would use the Henderson-Hasselbalch equation, pH = pKa + log[(base)/(acid)]\
HPO4= + H+ ==> H2PO4-
38 mL HPO4= x 0.650M = 24.7 mmoles.
Determine the volume (in mL) of 0.844 M hydrochloric acid (HCl) that must be added to 38.0 mL of 0.650 M sodium hydrogen phosphate (Na2HPO4) to yield a pH of 6.92.
Assume the 5% approximation is valid and report your answer to 3 significant figures. Look up pKa values.
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