The next three questions pertain to the preparation of a buffer starting with th

Question

The next three questions pertain to the preparation of a buffer starting with the salt of the weak base and adding an amount of strong acid to convert some salt to its weak acid. The amount of strong acid is selected to achieve the desired amount of the conjugate pair concentration ratio at the specified pH. You need to prepare 100 mL of a solution that is 0.05 M sodium bicarbonate. Calculate amount sodium bicarbonate you need to weigh out to prepare this solution. 3. You want to convert a portion of the bicarbonate in this solution to carbonic acid with a molarity of 0.02M utilizing I M HCI. How much HCI do you add? (Ignore the slight change in volume.) 4. 5. What will be the molarities of the two components in the solution? 6. Write three acid dissociation chemical equations for phosphoric acid. 7. If you are preparing a phosphate buffer with a pH of 12.2, which chemical equation are you working with and what are the two components of the buffer?

Explanation / Answer

Ans. #3. Required Moles of NaHCO3 = Molarity x Volume of solution in liters

                                                            = 0.05 M x 0.100 L

                                                            = 0.005 mol

Now,

            Required mass of NaHCO3 = Required moles x MW

                                                            = 0.005 mol x (84.007 g/ mol)

                                                            = 0.420035 g

#4. Being strong electrolyte, NaHCO3 undergoes complete dissociation in aqueous solution as follow-            NaHCO3 ---------> Na+ + HCO3-

# The resultant bicarbonate ion reacts with H+ from HCl to form carbonic acid as follow-

            HCO3-(aq) + H+(aq) ----------> H2CO3(aq)

# Given, [H2CO3] to be formed = 0.02 M

Now,

            Moles of H2CO3 to be formed = 0.02 M x 0.100 L = 0.002 mol

Following stoichiometry, 1 mol HCl forms 1 mol H2CO3.

So,

            Required moles of HCl = 0.002 mol

Now,

            Required vol. of HCl = required moles / Molarity =

                                                0.002 mol / 1M = 0.002 L = 2.000 mL

#5. Moles of H2CO3 formed = 0.002 mol

So,       [H2CO3] = 0.002 mol / 0.100 L = 0.02 M

# Remaining moles of HCO3- = Initial moles – Moles of formed

                                                = 0.005 mol – 0.002 mol = 0.003 mol

Now,   [HCO3-] = 0.003 mol / 0.100 L = 0.003 M

#6. 1st dissociation: H3PO4 + H2O Û H3O+ + H2PO4- ; Ka1 = 7.10 x 10-3

2nd dissociation: H2PO4- + H2O Û H3O+ + HPO42- ; Ka2 = 6.30 x 10-8

3rd dissociation: HPO42- + H2O Û H3O+ + PO43- ; Ka3 = 4.20 x 10-13

#7. The 3rd dissociation reaction is opted for preparing phosphate buffer of pH 12.2 because the buffering capacity is maximum when the pKa of the weak acid is closets to the pH of the buffer- in this case pKa of HPO42- = -log Ka = -log 4.20 x 10-13 = 12.38

# The two components of buffer are-

            Weak acid = HPO42-

            Conjugate base = CO32-

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