0.1000 M Ag^+ 0.1000 M Hg2^2+ 10.00 mL 0.1000 M KCN Hg2(CN)2 AgCN

0.1000 M Ag^+ 0.1000 M Hg2^2+ 10.00 mL 0.1000 M KCN Hg2(CN)2 AgCN

 

 

A mixture having a volume of 10.00 mL and containing

0.1000 M Ag^+ and 0.1000 M Hg2^2+ was titrated with

0.1000 M KCN to precipitate Hg2(CN)2 and AgCN.

a) Calculate pCN^- at each of the following volumes of added KCN:

5.00, 10.00, 15.00, 19.90, 20.10, 25.00, 30.00, 35.00 mL.

b) Should any AgCN be precipitated at 19.90 mL?

 

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> Hg2(CN)2의 Ksp = 5.0×10^(-40)

> AgCN의 Ksp = 2.2×10^(-16)

---> Ksp 값이 더 작은 Hg2(CN)2가 먼저 침전.

 

 

 

Hg2^2+(aq) + 2CN^-(aq) → Hg2(CN)2(s)

 

Ag^+(aq) + CN^-(aq) → AgCN(s)

 

 

 

(0.1000 mmol/mL) (10.00 mL) = 1.000 mmol Hg2^2+

 

 

 

a) 5.00 mL

(0.1000 mmol/mL) (5.00 mL) = 0.500 mmol CN^-

 

 

 

Hg2^2+(aq) + 2CN^-(aq) → Hg2(CN)2(s)

 

Hg2^2+ : CN^- = 1 : 2 계수비(= 몰수비) 이므로,

CN^-가 한계 반응물이고,

0.500 mmol CN^-와 반응하는

Hg2^2+의 몰수 = 0.250 mmol

 

 

 

남아있는 Hg2^2+의 몰농도를 계산하면,

[Hg2^2+] = 0.750 mmol / 15.00 mL = 0.0500 M

 

 

 

Ksp = [Hg2^2+] [CN^-]^2

 

[CN^-]^2 = Ksp / [Hg2^2+]

 

[CN^-] = (Ksp / [Hg2^2+])^(1/2)

= (5.0×10^(-40) / 0.0500)^(1/2)

= 1.00×10^(-19) M

 

 

 

pCN^- = –log(1.00×10^(-19)) = 19.0

= 19.00