Question

To make a phosphate buffer at pH 6.82 starting with one liter of 20 mM phosphoric acid at pH 2.15 (H₃PO₄; pKs are of 2.15, 6.82, and 12.38), you could add:

a) NaOH
b) HCl
c) Na₂HPO₄
d) NaH₂PO₄

Answer 1

To create a phosphate buffer at pH 6.82, sodium monohydrogen phosphate (Na₂HPO₄) should be added to the phosphoric acid, leveraging the weak base/conjugate acid buffer system at the desired pH level.

Step-by-step explanation:

To make a phosphate buffer at pH 6.82 starting with one liter of 20 mM phosphoric acid at pH 2.15, you could add sodium monohydrogen phosphate (Na₂HPO₄). Phosphoric acid is a triprotic acid with pKa values of 2.15, 6.82, and 12.38. At a pH near the second pKa (6.82), the predominant species are HPO₄²⁻ (the conjugate base) and H₂PO₄⁻ (the weak acid). To establish a buffer at pH 6.82, one must use a salt of the conjugate base, which in this case is Na₂HPO₄. When Na₂HPO₄ is added to the solution, it will react with the H₃PO₄ to form more of the H₂PO₄⁻, thereby establishing a buffer system where the ratio of the weak base to its conjugate acid is suitable for the desired pH.