Question

Consider how best to prepare one liter of a buffer solution with
pH = **8.09** 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 **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 =

Answer #1

Q1.

nearest pKa value to the pH is that of H2PO4- and HPO4-2

H2PO4- HPO42- with KA = 6.2 x 10-8 or pKa = 7.21

so the buffer equation

pH = pKa + log(HPO4-2 / H2PO4-)

Q2.

V = 1 L, M = 1 weak base

mmol of HPO4-2 = MV = 1 mol

pH = pKa + log(HPO4-2 / H2PO4-)

8.09= 7.21+ log(1 / H2PO4-)

1/ mol of H2PO4- = 10^(8.09-7.21) = 7.585

mol of H2PO4- = 1/7.585 = 0.13183 mol

now

we can get this from:

NaH2PO4 and N2HPO4

mol of NaH2PO4 = 0.13183

mass = mol*MW = 0.13183 *119.98 = 15.816 g

mol of Na2HPO4 = 1

mass = mol*MW = 1*141.96 = 141.96 g

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