lab 2

Question 1

Group II properties?

Answer 1

• group reagent: H₂S in presence of HCL
• ions Cu⁺⁺, Cd⁺⁺
• formulae of ppts.: CuS, CdS
• distingushing features: percpitate as sulfates in acid medium

Question 2

Precipitation of group II cations takes place in the presence of 0.25 M HCl

Why?

Answer 2

H₂S ⇔ 2H⁺ + S2^-

HCl → H⁺ + Cl^-

The higher hydrogen ion conc. causes the first equation to shift backwards; decreasing the conc. of sulphide ions enough to exceed the K_sp of group II cations only.

Question 3

Q. What happens if the concentration of HCl used was lower? 0.20 M?

Answer 3

The [S^2-] will be high enough to precipitate group IV together with group II cations.

Question 4

Q. What if the concentration used is higher than 0.25 M HCl? 0.30 M?

Answer 4

The dissolution of H₂S will be further inhibited leading to sharp decrease in the conc. of [S^2-] thus [S^2-] won’t be sufficient to precipitate sulfides of group II completely.