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Phosphate System and Physiological pH

The phosphate system serves to buffer the intracellular fluid of cells at physiological pH because pK2 (7.20) lies near the normal pH range of 6.9-7.4. If the total cellular concentration of phosphate is 19mM and the pH is 6.9, what is the concentration of (H2PO4-) in mM??

Is this correct???
H2PO4-(aq) H+(aq) + HPO42-(aq)

Ka = [H +] [HPO42-]
[H2PO4-]

Ka for this equilibrium is 6.23 × 10-8 at 25°C.

Can I not just use this Ka and use -log (6.9) for [H] and use 19mM as the concentration of [HPO42-]??

Solution Preview

Question: Can I use 19mM as the concentration of [HPO42-]?

Answer: Not exactly. The [H2PO4-] + [HPO4^-2] is 19 mM, not just [HPO4^-2]. So, what we have to do is subtract the [HPO4^-2] and then we've got the right answer. ...

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