Phosphate Buffering System - Clarification

[127330]

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Perhaps I'm overlooking something so simply, but I cannot figure out how my Biochemistry book came up with this specific answer:

[H2PO4^-2]/[HPO4^1-] ratio: 1.58

[HPO4^2-]+[H2PO4^1-] = 20mM

Thus, the book says:

[HPO4^2-] = 12.25 mM
[H2PO4^1-] = 7.75 mM

What the heck am I doing wrong?

20 mM x (1/1.58) = 12.65 mM

 

[ashtonjam]

When you're doing ratios like that, remember to add 1 and 1.58 together. The total is 2.58.

This means H2PO4 is 1.58/2.58 and HPO4 is 1/2.58. Multiply 1.58/2.58 and 1/2.58 by 20 mM and you get your answers.