# What volumes of 50 mM #CH_3COOH# and 30 mM #NaCH_3COO# solutions are needed to prepare 2L of 20 mM acetate buffer pH 3.6? (#pK_a# for acetic acid is 4.74)

##### 1 Answer

#### Answer:

**WARNING! Long answer!** You need to mix 750 mL of acetic acid with 90 mL of sodium acetate and add enough water to make 2 L of buffer.

#### Explanation:

The strategy to follow is

1) Write the **chemical equation** for the buffer.

2) Calculate the **concentration ratio** of

3) Calculate the **molar ratio** of

4) Calculate the **moles** of

4) Calculate the **volumes** of

**1) Chemical equation**

**2) Calculate the concentration ratio**

We use the Henderson-Hasselbalch equation to calculate the concentration ratio.

**3) Calculate the molar ratio**

Since both the acetic acid and the sodium acetate are in the same solution, the ratio of their concentrations is the same as the ratio of their moles. That is,

**4) Calculate the moles**

∴

**5) Calculate the volumes**

Thus, you mix 750 mL of the acetic acid with 90 mL of the sodium acetate and add enough water to make 2 L of pH 3.6 buffer.