Answer to Question #303798 in Physical Chemistry for Chemistry

Question

In a Laboratory, you have the following chemicals available:

1.0 M NaOH, 1.00 M HCl, 1.00 M H3PO4, NaH2PO4 (s), Na2HPO4 (s), and Na3PO4 (s).

Describe TWO ways of making 1000 mL of a buffer of pH = 12 (Use only the above

chemicals, plus distilled water.)

[Given: H3PO4 - Ka1 = 6.9 x 10-3; NaH2PO4 - Ka2 = 6.2 x 10-8 ; Na2HPO4 - Ka3 = 4.8 x 10-13]

Answer 1

MATERIALS

Reagents

Na2HPO4•2H2O

NaH2PO4•H2O

METHOD

1. Dissolve 35.61 g of Na2HPO4•2H2O and 27.6 g of NaH2PO4•H2O separately in H2O.

2. Adjust the volume of each solution to 1000 mL.

Store the stock solutions for up to 6 mo at 4°C.

3. To prepare the buffer, mix the stock solutions as follows:

o i. For 50 mL: 38.5 mL of Na2HPO4•2H2O and 11.5 mL of NaH2PO4•H2O.

o ii. For 100 mL: 77 mL of Na2HPO4•2H2O and 23 mL of NaH2PO4•H2O.

o iii. For 500 mL: 385 mL of Na2HPO4•2H2O and 115 mL of NaH2PO4•H2O.

o iv. For 1000 mL: 770 mL of Na2HPO4•2H2O and 230 mL of NaH2PO4•H2O.

4. Check the pH of the solution at room temperature. If necessary, adjust the pH to 7.3