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 | K_{a} |
pK_{a} |
---|---|---|---|

HC |
C |
6.4 x 10 |
4.19 |

H |
HPO |
6.2 x 10 |
7.21 |

HCO |
CO |
4.8 x 10 |
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)} = 5.37x10^{6}

ratio 2 = 10^{(10.92-7.21)} = 5128.61

ratio 3 = 10^{(10.92-1032)} = 3.98

So the system CO_{3}^{2-} /
HCO_{3}^{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 = [CO_{3}^{2-}] /
[HCO_{3}^{-}]

[HCO_{3}^{-}] = 1 / 3.98 = 0.25 M

Finally, the mass for these compounds would have to be:

mHCO_{3}^{-} = 0.25 mol/L * 1 L * (39+1+12+48) = 25
g

mCO_{3}^{2-} = 1 * 1 * (2*39+12+48) = 132 g

Hope this helps

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