2. Buffers. Physiological pH inside of living cells is about 7.4. The total free
ID: 979912 • Letter: 2
Question
2. Buffers. Physiological pH inside of living cells is about 7.4. The total free concentration of phosphate inside of cells is about 1 mM. Phosphate exists in four different protonation states (see the figure below, taken from your textbook). lb n bul .onu H+ H+ HPo2- Po,3 PO.3- pKa= 2.12 pKa = 7.21 pKa= 12.67 Unnumbered 1 p1 Blechemistry, Seventh Edition 2012 W. Freeman a and Company A) Which form of phosphate is the most abundant at physiological pH? B) Suppose that 100 ml of cellular extract is treated with 50 ml of 0.5 mM HC1. What is the final pH? [Assume that the HCl reacts only with the phosphate buffer in the cell extract.] Which form of phosphate is most abundant now?Explanation / Answer
A) At physiological pH the form of phosphate is HPO4^2-.
B) moles of HCl added = 0.5 mM x 0.05 L/1000 = 2.5 x 10^-5 mols
New moles HPO4^2- = 0.001 M x 0.1 L - 2.5 x 10^-5 = 7.5 x 10^-5 mols
New moles H2PO4- = 0.001 M x 0.1 L + 2.5 x 10^-5 = 1.25 x 10^-4 mols
[HPO4^2-] = 7.5 x 10^-5/0.15 = 5 x 10^-4 M
[H2PO4-] = 1.25 x 10^-4/0.15 = 8.33 x 10^-4 M
For buffer,
pH = pKa + log([base]/[acid])
= 7.21 + log(5 x 10^-4/8.33 x 10^-4)
= 6.99
Most abundant form H2PO4-
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