Question

To make a phosphate buffer at pH 6.82 starting with one liter of 10 mM phosphoric acid (H3PO4; pKs are of 2.15, 6.82, and 12.38), you could add

a) NaOH
b) HCl
c) NaH₂PO₄
d) Na₂HPO₄

Answer 1

To achieve a phosphate buffer at pH 6.82, sodium monohydrogen phosphate (Na₂HPO₄) should be added to phosphoric acid, forming a buffer system with the weak acid and its conjugate base.

Step-by-step explanation:

To make a phosphate buffer at pH 6.82 starting with one liter of 10 mM phosphoric acid, you would add Na₂HPO₄ (sodium monohydrogen phosphate). This is because phosphoric acid (H₃PO₄) has a pKa value of 6.82 at its second dissociation step, corresponding to the equilibrium between the dihydrogen phosphate ion (H₂PO₄⁻) and the hydrogen phosphate ion (HPO₄⁻⁲). Adding Na₂HPO₄ to H₃PO₄ will create a buffer system containing both the weak acid (H₂PO₄⁻) and its conjugate base (HPO₄⁻⁲), which is necessary to resist changes in pH.