a. Buffer 1: 30 mL of sodium dihydrogen phosphate and 20 mL sodium hydrogen phos

Question

a. Buffer 1: 30 mL of sodium dihydrogen phosphate and 20 mL sodium hydrogen phosphate

You will determine the theoretical pH of the buffer systems you will be creating in lab. Making the pH Buffers before completing these questions. Record your answers in your lab notebook for later comparison. The concentrations of both NaH_2 PO_4 and Na_2 HPO_4 is 0.50 M. Using the information provided in the procedure, determine the theoretical pH of Buffer 1. Assume you will make the solution in a 100 mL volumetric flask. Also assume that the volumes of each component are exactly as stated in the procedure (which will not be the case during your experiment). Using the information provided in the procedure, determine the theoretical pH of Buffer 2. Assume you will make the solution in a 100 mL volumetric flask. Also assume that the volumes of each component are exactly as stated in the procedure (which will not be the case during your experiment).

Explanation / Answer

H2PO4- + H2O <---> HPO4-2 + H3O+

               Ka = [HPO4-2][H3O+]/[H2PO4-] = 6.23 x 10^-8
               pKa = 7.21
              
pH = pKa + log [HPO4-2]/[H2PO4-]
PART A:
we have for buffer 1: 30 mL of 0.5M NaH2PO4 and 20 mL of 0.5M Na2HPO4
Therefore,   
[HPO4-2 ] = 0.5 M x 20 mL / 100 mL = 0.1 M
[H2PO4-] = 0.5 M x 30 mL/100mL = 0.15 M

pH = pka + log [HPO4-2]/[H2PO4-]
   = 7.21 + log (0.1/0.15)
   = 7.03

PART B:
Similarly, for buffer 2: we have 20 mL of 0.5M NaH2PO4 and 30 mL 0.5M Na2HPO4

Therefore,
[HPO4-2 ] = 0.5 M x 30 mL/100mL = 0.15 M
[H2PO4-] = 0.5 M x 20 mL / 100 mL = 0.1 M

pH = pka + log [HPO4-2]/[H2PO4-]
   = 7.21 + log (0.15/0.10)
   = 7.39
   

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