Consider how best to prepare one liter of a buffer solution with pH = 6.55 using

Question

Consider how best to prepare one liter of a buffer solution with pH = 6.55 using one of the weak acid/conjugate base systems shown here.

HC2O4-

C2O42-

6.4 x 10-5

4.19

H2PO4-

HPO42-

6.2 x 10-8

7.21

HCO3-

CO32-

4.8 x 10-11

10.32

How many grams of the sodium salt of the weak acid must be combined with how many grams of the sodium salt of its conjugate base, to produce 1.00 L of a buffer that is 1.00 M in the weak base?

grams sodium salt of weak acid =

grams sodium salt of conjugate base =

Weak Acid Conjugate Base Ka pKa

HC2O4-

C2O42-

6.4 x 10-5

4.19

H2PO4-

HPO42-

6.2 x 10-8

7.21

HCO3-

CO32-

4.8 x 10-11

10.32

Explanation / Answer

This problem is similar to the one I answered earlier but with different data, so, use the same procedure but change the data. Here's the sample:

To determine the best buffer combination, calculate first the ratio (conjugate base/acid) which is calculated as:

ratio = 10pH-pKa

The smallest number is the best buffer to use. The other way to solve it, it's matching the pH value to the pKa closer to that pH. In this case the HPO42-/H2PO4- with a pKa of 7.21 is the best for the buffer.

Now, as we know that the weak base has to be 1 M, then the acid would have to be:
ratio = [HPO42-]/[H2PO4-]
106.55-7.21 = [HPO42-]/[H2PO4-]
[HPO42-]/[H2PO4-] = 0.2187 ------> [H2PO4] = 1 / 0.2188 = 4.57 M

Finally, the mass for these compounds would have to be:
mH2PO4- = 4.57 * 1 * (23+2+31+64) = 548.4 g
mHPO42- = 1 * 1 * (2*23+1+31+64) = 142 g

Hope this helps

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