Recall the acid strength exercise in the Acids, Bases, and pH lab. All acid solu

Question

Recall the acid strength exercise in the Acids, Bases, and pH lab. All acid solutions were 0.001M, but did not all have a pH of 3; some were weak acids, in which some of the acid remained undissociated and some dissociated to form H3O+ and the conjugate base. In these solutions, the total concentration of both the undissociated acid and the conjugate base is 0.001M. If you need to prepare a 0.1M phosphate buffer solution with pH 4, what will the total concentration of phosphate species (ions) be? If x is the concentration of conjugate base in the buffer solution, what is the concentration of the acid?

Explanation / Answer

From Henderson's equation:

pH = pKa + log[salt]/[acid]

pKa,1 of phosphoric acid = 2.16, pKa,2 of phosphoric acid = 7.21, pKa,3 of phosphoric acid = 12.32

pH of the solution is 4, which is closest to pKa,1. The two species present in the solution is H3PO4 and H2PO4-.

Let [H2PO4-] = x,

So, [H3PO4]= 0.1 - x

pH = pKa + log (x / 0.1 -x)

or, 4 = 2.16 + log (x / 0.1 -x)

or, (x / 0.1 -x) = 69.183

or, x = [H2PO4-] = 0.0986 M

and, [H3PO4]= 0.1 - x

= 1.42 * 10-3 M

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