Question

7. Consider the weak acid base reaction,

KH₂PO₄ --> HPO₄^-2 + H⁺

7a. To make a pH 7.0 buffer, what concentration of HPO₄^-2 would be needed if the concentration of KH₂PO₄ is 0.1M (see Figure 2-18 in your book).

7b. To make a pH 7.0 buffer, what concentrations of HPO₄^-2 and KH₂PO₄ would be required assuming a the total phosphate molarity ([HPO₄^-2] + [KH₂PO₄]) is 0.3 M.

Answer 1

7) The buffer is made of H2PO4-(acid) and HPO4-2 (conjugate base)

Ph of buffer = 7.0 and [H2PO4-] = 0.1M [HPO4-2] = ?

We know pH of a buffer is calculated using Hendersen equation

pH = pKa + log [HPO4-2]/[H2PO4-]

7.0 = 7.2 + log [HPO4-2]/0.1

Solving we get

[HPO4-2] = 0.063M

7b)

Now the total molarity of [HPO4-2] + [H2PO4-] = 0.3

Let [HPO4-2] be x M then [H2PO4-] = 0.3-x

Then pH = 7.0 = 7.2 + log x/(0.3-x)

solving for x , we get

x = 0.116 M

Thus [HPO4-2] = 0.116M and [H2PO4-] = 0.3-0.116 = 0.184