Question

1)         A liter of 0.050M pH 7.10 potassium phosphate buffer is needed for a certain experiment.

            The following four methods were proposed for the preparation of the buffer. Using the pK_a

            value given in your textbook, which of these methods should be used and what is wrong

            with the other proposals? Letter of method which should be used: __________________________

(A)      A 0.050M sample of K₂HPO₄ was dissolved in about 800.0mL of water, and the pH was

                        adjusted to 7.10 using a standardize pH meter and a 1.00M HCl solution, followed by

                        addition of water to a volume of 1.00L.

(B)       A 0.050M sample of K₂HPO₄ was dissolved in about 800.0mL of water, and the pH was

                        adjusted to 7.10 using a standardize pH meter and a 1.00M H₃PO₄ solution, followed by

                        addition of water to a volume of 1.00L.

(C)       Use the Henderson-Hasselbalch equation and 0.050moles = “X” moles of KH₂PO₄ and

                        “Y” moles of K₂HPO₄ to calculate the moles of the two salts, weigh out the salts,

                        dissolve, and dilute to 1.00L.

(D)      A 0.050M sample of KH₂PO₄ was dissolved in about 800.0mL of water, and the pH was

                        adjusted to 7.10 using a standardize pH meter and a 1.00M KOH solution, followed by

                        addition of water to a volume of 1.00L.

Answer 1

The buffer action of potassium phosphate buffer is due to inter conversion of the acid  H2PO₄^- to the salt HPO₄^2-.

Hence a potassium phosphate buffer contains KH2PO4 and K2HP4.

H2PO₄^-(aq) + H2O(l) ------> HPO₄^2- (aq) + H3O⁺  , pKa = 7.21

(A) Here we are taking only K2HPO4 and HCl. Since both are acids we are unable to get the desired concentration of KH2PO4 required to make the buffer. Also we will be unable to produce a basic pH of 7.1. Hence this cannot the best method.

(B) Here we are taking only K2HPO4 and H3PO4. Since both are acids we are unable to get the desired concentration of KH2PO4 required to make the buffer. Also we will be unable to produce a basic pH of 7.1. Hence this cannot the best method.

(C) This method contains both KH2PO4 andK2HPO4 of desired concentration. However we will be unable to get exact pH of 7.1 if there is any error in the measurement of the required amount of the salt. We can use this method but not the best one.

(D) Here we are taking KH2PO4 and KOH. KOH will react with KH2PO4 to form K2HPO4. Hence we can get the desired concentration of KH2PO4 and K2HPO4. Also we can get an exact basic pH of 7.1 by adjusting the KOH solution. Hence method D should the the best method.