Phosphoric acid buffer: Acid addition : Initially, a 1.00liter buffer solution c

Question

Phosphoric acid buffer: Acid addition : Initially, a 1.00liter buffer solution contains H2PO4 and HPO4 2 at pH = 6.78. The pKa for this H2PO4 / HPO4 2 buffer is 7.20.

Then, hydrochloric acid (HCl) gas is bubbled through the buffer solution. As a result, 0.0423 moles of HCl are added without changing the volume of the solution. Since HCl is a strong acid, it fully dissociates into H+ and Cl . This causes the solution pH to decrease by 0.20 pH units after the reaction between added H+ and the buffer system is complete.

9a. Write down the chemical reaction taking place between the H2PO4 / HPO4 2 buffer system and the hydrochloric acid (H+ ) in solution. Make sure to balance the stoichiometry of the equation.

9b. What were the original concentrations of H2PO4 and HPO4 2 in solution before the bubbling started? Hint: Try to set up equations describing the system conditions before and after bubbling with two HendersonHasselbach equations. Think about how the acid and base concentrations of the buffer system changed due to the reaction with HCl (H+ )

SHOW STEP BY STEP PLEASE!

Explanation / Answer

for the buffer solution,

9a. Equations,

HPO4^2- + H+ ----> H2PO4-

This increases concentration of H2PO4- in buffer solution and decreases concentration of HPO4^2- in solution

9b. Before addition of HCl,

pH = pKa + log(HPO4^2-/H2PO4-)

6.78 = 7.20 + log(HPO4^2-/H2PO4-)

[HPO4^2-] = 0.38[H2PO4-] ... (1)

After addition of HCl

6.58 = 7.20 + log(HPO4^2-/H2PO4-)

[HPO4^2-]/[H2PO4-] = 0.24 ----(2)

From (1) and (2),

(0.38[H2PO4-])/[H2PO4-] = 0.24

Initial concentrations are,

[H2PO4-] = 0.63 M

So, [HPO4^2-] = 0.24 x 0.63 = 0.15 M

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