a) If a salt containing the conjugate base of a weak acid is dissolved in water,

ID: 524682 • Letter: A

Question

a) If a salt containing the conjugate base of a weak acid is dissolved in water, the pH of the resulting solution is _____ (less than, equal to, greater than) seven. b) The K_sp of Ag_2CO_3 is 8.1 times 10^-12. If you want to increase the concentration of Ag^+ in a saturated solution of Ag_2CO_3, you would add _____ (NaOH, HNO_3, NaCO_3 or AgNO_3). c) The K_a of the weak acid HA is 7.2 times 10^-6. The pH of a solution prepared by mixing 10 mLs of 0.1M NaA is _____ (4.1, 4.8, 5.1, 6.8). d) The pH at the equivalence point of a titration of a weak base by a strong acid will be _______ (less than 7, equal to 7, greater than 7, equal to the pK_b of the base). e) Given the following Ksp's: BaSO_4 K_sp = 1.1 times 1-^-10; CaSO_4 K_sp = 2.4 times 10^-5; PbSO_4 K_sp = 6.3 times 10^-7. If a solution of Na_2SO_4 is added dropwise to a solution containing Ba^2+, Ca^2+ and Pb^2+, the first species to precipitate is _____ (Na_2SO_4, BaSO_4, CaSO_4, PbSO_4).

Explanation / Answer

a) The conjugate base (A-) of a weak acid (HA) will be strong in nature.

HA <-------> H+ + A-

Thus when the salt of this strong base (NaA) is added to water, NaA completely dissociates in water. The A- ion abstracts protons from water to result in the formation of OH- ions. This increases the pH of the solution. Hence the solution will become basic

NaA -----> Na+ + A-

A- + H2O <------> HA + OH-

b) The common ion between Ag2CO3 and AgNO3 is Ag+. Thus to increase the [Ag+] will cause the equilibrium of Ag2CO3 to move to solid side and more Ag2CO3 will be precipitated out. Similar common ion effect will be observed with Na2CO3. Addition of NaOH will precipitate Ag+ ions as AgOH thus decreasing its concentration in the solution.

H+ ions of HNO3 reacts with the carbonate ions to form carbonic acid H2CO3. Carbonic acid decomposes to CO2 and H2O. This decreases the amount of CO32- ions in the solution thus increasing the ionic product (Qsp) of the solution i.e more Ag+ ions will go in the solution. Hence HNO3is the correct answer.

c) HA <-------> H+ + A-

Ka = [H+] [A-] / [HA]

Initially A- and HA are present. The change in the [H+] can be calculated using ICE table

[H+]

[A-]

[HA]

Initial

0.1 M

Change

-x

Equilibrium

0.1-x

Since x is very small quantity 0.1-x = 0.1

7.2 X 10-6 = x (0.1) / 0.1

x= 7.2 X 10-6

[H+]= 7.2 X 10-6

pH = -log 7.2 X 10-6 = 5.14

e) Referring to Ksp values of CaSO4, BaSO4 and PbSO4, we know that CaSO4 is most soluble and BaSO4 is least soluble salt out of the three. Hence, BaSO4 will precipitate first.