You are instructed to create 500. mL of a 0.65 M phosphate buffer with a pH of 7

Question

You are instructed to create 500. mL of a 0.65 M phosphate buffer with a pH of 7.2. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4available. (Enter all numerical answers to three significant figures.)

Ka1 = 6.9103

Ka2 = 6.2108

Ka3 = 4.81013

The acid component of the buffer is NaH2PO4. The base component of the buffer is Na2HPO4.

1. What is the molarity needed for the acid component of the buffer? (in M)
2. What is the molarity needed for the base component of the buffer? (in M)
3. How many moles of acid are needed for the buffer? (in mol)
4. How many moles of base are needed for the buffer? (in mol)
5. How many grams of acid are needed for the buffer?
6. How many grams of base are needed for the buffer?

H3PO4(s) + H2O(l) H3O+(aq) + H2PO4(aq)    

Ka1 = 6.9103

H2PO4(aq) + H2O(l) H3O+(aq) + HPO42(aq)    

Ka2 = 6.2108

HPO42(aq) + H2O(l) H3O+(aq) + PO43(aq)

Ka3 = 4.81013

Explanation / Answer

1) the pH required = 7.2

pKa = 7.2 ( Ka2)

pH = pKa2 + log [salt] / [acid]

so concentration of acid = concentration of base component

1) molarity of acid = 0.65 M

2) molarity of base = 0.65

3) Volume of buffer = 500mL 0.5 L

Moles = Molarity X volume = 0.65 X 0.5 = 0.325 moles

4) 0.325 moles as well

5) 1 mole = 120 grams [NaH2PO4]

0.325 moles = 120 X 0.325 = 39 grams

6) 1 mole = 142 grams[Na2HPO4]

0.325 moles =142 x 0.325 = 46.15 grams

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