20)What is the best way to ensure complete precipitation of SnS from a saturated

Question

20)What is the best way to ensure complete precipitation of SnS from a saturated solution? a) Add a weak base. b) Add a strong base. c) Add H2S. d) Add a weak acid. e) Add a strong acid. 21)Which of the following is true for a buffered solution? a) The solution will not change its pH very much even if a strong base is added. b) Any H ions will react with a conjugate base of a weak acid already in solution c) The solution will not change its pH very much even if a concentrated acid is added. d) The solution resists change in its (H1) All of these 22)Sodium chloride is added slowly to a solution that is 0.010 M in Cu , Ag, and Au. The Ksp values for the chloride salts are 1.9 x 10-7, 1.6 x 10-10, and 2.0x 10-13, respectively. Which compound will precipitate first? a) AgCI b) AuCI c) CuCI d) All will precipitate at the same time. e) Cannot be determined. 23)An industrial plant processes its waste water through a sedimentation tank that removes hazardous metals by precipitating them as insoluble carbonate salts. If sodium carbonate is gradually added to the tank, what would be the order of precipitation of the metals, Pb2 Cu2*, Hg22+, and Zn2. if each is 1.0 x 10-4 M? (Ksp PbC0?? 7.4 x 10-14, Ksp CuCOjz 1.4 x 10-10. Ksp HgzCO3 8.9 x 10-17, and Ksp ZnCO 1.4x 10-11) a) All metal ions will precipitate at the same time. b) Pb2, Hga2, Zn2*, Cu2 c) Cu, Zn2, Hg, Pb2 e) Cu, Zn2, Pb2, Hg The following data are for Questions 24 and 25 Mg(OH)2 CaF2 Pblz PbBrz Ca(OH)2 Ksp 1.5 x 10-11 3.9 x 10-11 8.7 x 10-9 6.3 x 106 7.9 x 106 24)Which salt has the highest molar solubility in water? b) CaF2 c) Pblz d) PbBrz e) Ca(OH)2 a) Mg(OH)2 25)Which salt has the next to the lowest molar solubility in water? b) CaF2 c) Pblz d) PbBrz e) Ca(OH)2 a) Mg(OH)2

Explanation / Answer

Q20) SnS dissociates in solution into Sn2+ and S2- ions. To precipitate SnS from its saturated solution H2S must be added, as H2S dissociates to release H+ and S2- ions,and due to common ion effect of S2- ions(n excess) ,the following equilibrium will shift in reverse direction.This happens to conteract the change as the change in S2- ion concentration disturbs the equilibrium.(Le Chatlier's principle).As the equilibrium shifts in reverse direction , SnS is precipitated.When H2S is added in excess,the reaction completes in reverse direction and complete precipitation takes place.

SnS(s) <---->Sn2+(aq) +S2-(aq)

H2S <-->2H+ +S2-

Answer: c) Add H2S

Q21) e) All of these

pH=-log [H+]

So, a buffer can be defined as a solution of a weak acid and its conjugate base that resists any change in pH or [H+],when strong acid or strong base is added to it.

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