Question

In our experiment, we will be using a portion of the phosphate buffer system that is based upon the following equilibrium: H2PO4- HPO42- + H+ pKa = 7.2

In this case, H2PO4- will act as the acid and HPO42- will act as the base.

Materials:

1M NaOH: 40.01 g/L of solution

1M HCl: 83 mL conc. HCl/L of solution

Potassium phosphate, dibasic, K2HPO4, MW= 174.18

Potassium phosphate, monobasic, KH2PO4 MW= 136.09

**I already preformed this lab, but I struggled a lot during the lab and would appreciate guidance on the procedure.**

Experimental Procedure PART 1

In this experiment, you are asked to make buffer solutions of phosphate using three different methods. Your TA will assign the desired pH for each of your buffer solutions (5.8, 6.5, 7.4) . Make 50 mL of solution containing 0.1 M final concentration of the buffer. When you have finished your calculations, prepare the solutions and check the pH of your buffers using a pH meter.

1. Use Henderson-Hasselbalchj equation to calculate the ratio, [A-]/[HA]

2.Use the buffer concentration equation that relates conjugate base and conjugate acid concentrations to the total buffer concentration to calculate the concentration of each component of the buffer.

3.Use the volume of the buffer (50 mL) to calculate the number of moles of conjugate acid (HA) and conjugate base (A-) you need to prepare your buffers.

4.How you proceed from here will depend on which buffer you are preparing and which method you use.

5.Use the molecular weight (see useful information in the materials section) to calculate the amount (grams) of each solid buffer component. Use the concentrations of stock solutions and the dilution equation to calculate the volume of required stock solutions.

6.Measure out and mix the two components, then adjust the volume to 50 mL to give a final buffer concentration of 0.1 M. Measure and note the pH of each buffer using a pH meter. (5.60, 6.10, 7.01) pH after checking with the pH meter

7.Investigation of buffer capacity. Select two of your buffer solutions (it is advisable to use the two solutions that were closest to your assigned pH value). Investigate the buffer capacity of your buffer against both acid and base by adding 0.5 ml of 1.0 M HCl to 50 mls of your buffer. Mix well and check the pH, noting the difference from your original buffer. Repeat this procedure with 0.5 ml of 1.0 M NaOH and your second buffer. Use the method given during recitation to determine the theoretical changes in the pH and compare to the actual changes.

Answer 1