How many moles would you need to make the buffer? Suppose you are working in a B

Question

How many moles would you need to make the buffer?

Suppose you are working in a Biochemistry lab and the pH meter is broken. You urgently need to make a phosphate buffer at pH 7.8. From an inspection of your chemical inventory you note that you have the following forms of phosphate to make your buffer: phosphoric acid (H3PO4), mono-basic sodium phosphate (NaH2PO4), di-basic sodium phosphate (Na2HPO4) and tri-basic sodium phosphate (Na3PO4). You then find in your Biochemistry textbook that the 3 pKaS for phorphoric acid are pKa1 = 2.15, pKa2 = 7.2 and pKa3 = 12.4 In order to make 1 L of a 0.1 M sodium phosphate buffer at pH 7.8 how many moles of which compounds would you mix together? Show all your work.

Explanation / Answer

buffer conc = 0.1M   =[H2PO4-]+[HPO42-] ,

pH = pka2 + log [HPO42-]/[H2PO4-]

7.8 = 7.2 + log [HPO42-]/[H2PO4-]

[HPO42-] = 2.389 [H2PO42-]

now by solving two equations we get

1 =( 2.389 + 0.1)[H2PO4-]

[H2PO4-] = 0.4 M

[HPO42-] = 0.6 M

vol = 1 liter , hence moles = M x 1

hence 0.4 moles of H2PO4- and 0.6 moles of HPO42- are moxed to get buffer

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