Answer to Question #140236 in General Chemistry for Jean Aquino

Question

How many grams of sodium acetate is needed to add to a 0.2M acetic acid to prepare a buffer of pH=5?

Answer 1

To answer this, we need to know the Ka or pKa for acetic acid. The normal value given for the pKa is 4.75. Note, this is the same as the desired pH. This becomes important.

This is a BUFFER problem. Most buffer problems can best be addressed using the Henderson Hasselbalch equation:

pH = pKa + log [salt]/[acid] MEMORIZE THIS.

We know the pH (5)

We know the pKa (4.75). 

We know the [acid] = 0.20 M

We can find [salt] ,i.e [sodium acetate]

When the pH = pKa as in this case, the ratio of [salt]/[acid] =1 because log 1 = 0 

So, [sodium acetate] = 0.20 M. This can be seen as follows:

4.75 = 4.75 + log [sodium acetate]/[0.20]

0 = log x/0.2

x = 0.2 M

So, to now calculate the "amount" in grams, you would convert 0.20 M to grams using the volume of 1000 ml (0.2 L)

0.20 mole/L x 0.2 L = 0.04 moles moles sodium acetate

0.04 moles sodium acetate x 82 g/mole = 3.3 g