The pKa values of H3PO4 are 2.12, 7.21 and 12.67. The pH of a phosphate buffer containing 0.2 M NaH2PO4 and 0.1 M Na2HPO4 is _______�

Question

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Answer 1

H₃PO₄ ==> H⁺ + H₂PO₄^-   pKa = 2.12

H₂PO₄^- ==> H⁺ + HPO₄^2-  pKa = 7.21

HPO₄^2- ==> H⁺ + PO₄^3-   pKa = 12.67

 

In this buffer, the weak acid is NaH₂PO₄, and the conjugate base is Na₂HPO₄.

Without spectator ions, you have H₂PO4^- and HPO₄^2-

pH = pKa + log [conjugate base]/[acid]

pH = 7.21 + log [0.1 M]/[0.2 M]

pH = 7.21 + (-0.30)

pH = 6.91