Question

Find an appropriate buffer system (include the weak acid/base and conjugate base/acid salt) to make 1.0 M buffer at pH 7.3.

Answer 1

We first need to choose a buffer system. Since the desired pH is 7.3, we choose a phosphate buffer because the range of a phosphate buffer is 5.8 to 8.0

The pKa value for H2PO4- is 7.2. This suits our needs.

The following reaction takes place:

H2PO4- ---> HPO42- + H+

So, for this buffer we use NaH2PO4 as the acid and Na2HPO4 as the salt of conjugate base.

From the Henderson Hasselbach equation we have:

pH = pKa + log([base]/[acid])

Putting values:

7.3 = 7.2 + log([base]/[acid]) ---(I)

Also it is given that buffer conc is 1 M.

So,

[base] + [acid] = 1 ---(II)

Solve (I) and (II) to get:

[base] = 0.56 M

[acid] = 0.44 M

Assuming that we make a 1 L buffer:

Moles of acid NaH2PO4 needed = Molarity*Volume = 0.44*1 = 0.44

Mass of acid NaH2PO4 needed = Moles*MW = 0.44*120 = 52.8 g

Moles of bse Na2HPO4 needed = Molarity*Volume = 0.56*1 = 0.56

Mass of acid Na2HPO4 needed = Moles*MW = 0.56*142 = 79.52 g

So,

Mix 52.8 g of NaH2PO4 and 79.52 g of Na2HPO4 into 1 L distilled water to make a phosphate buffer of pH 7.3