# How would you prepare 500 mL of a 0.01 mol/L buffer with pH 7.00 from 0.2 mol/L stock solutions of sodium dihydrogen phosphate and disodium hydrogen phosphate?

##### 2 Answers

See below.

#### Explanation:

You dilute 25 mL of each stock solution to 500 mL to get 0.01 mol/L solutions.Then you mix 300 mL of 0.01 mol/L

**Calculate the dilution of the stock solutions**

We can use the dilution formula

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∴ You dilute 25 mL of each of the stock solutions to 500 mL to prepare the 0.01 mol/L solutions for the buffer.

**Calculate the volumes of each solution needed**

The chemical equation for the buffer is

or

The Henderson-Hasselbalch equation is

Both solutions have the same concentration, so the ratio of the volumes is the same as the ratio of the molarities.

**(1)**

**(2)**

Substitute (1) into (2).

Mix 300 mL of 0.01 mol/L

**Note**: The answer can have only one significant figure, because that is all you gave for the volume, the concentrations, and the pH.

Here's another way to prepare the buffer.

#### Explanation:

Add 16 mL of 0.2 mol/L

**Calculate the moles of each component needed**

The chemical equation for the buffer is

or

The Henderson-Hasselbalch equation is

Both solutions have the same concentration, so the ratio of the volumes is the same as the ratio of the moles.

**(1)**

**(2)**

Substitute (1) into (2).

**Calculate the volumes of each component**

Add 16 mL of 0.2 mol/L

**Note**: I calculated the volumes to two significant figures, but only one significant figure is justified by your data.