Question

Consider how best to prepare one liter of a buffer solution with pH = 10.92 using one of the weak acid/conjugate base systems shown here.

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

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

grams potassium salt of weak acid =

grams potassium salt of conjugate base =

Answer 1

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

ratio = 10^pH-pKa

The smallest number is the best buffer to use.

Now, let's do this with every system:

ratio 1 = 10^(10.92-4.19) = 5.37x10⁶
ratio 2 = 10^(10.92-7.21) = 5128.61
ratio 3 = 10^(10.92-1032) = 3.98

So the system CO₃^2- / HCO₃^2- will be the best system to prepare the desired buffer.

Now, as we know that the weak base has to be 1 M, then the acid would have to be:
ratio = [CO₃^2-] / [HCO₃^-]
[HCO₃^-] = 1 / 3.98 = 0.25 M

Finally, the mass for these compounds would have to be:
mHCO₃^- = 0.25 mol/L * 1 L * (39+1+12+48) = 25 g
mCO₃^2- = 1 * 1 * (2*39+12+48) = 132 g

Hope this helps