lab 3

Question 1

group III properties?

Answer 1

• group reagent: NH₄OH in presence of NH₄Cl
• ions: Al³⁺, Fe³⁺, Cr³⁺
• formulae of ppts.: Al(OH)₃, Fe(OH)₃, Cr(OH)₃
• distinguishing features: precpiated by NH₄OH in presence of NH₄Cl

Question 2

should hydroxide ions concentration be kept high or low and why?

Answer 2

• Hydroxide ion concentration should be kept low; in order for group III cations only to precipitate (lower Ksp) compared to hydroxide salts in further groups
• NH4OH ⇔NH4 + + OH-

NH4Cl → NH4 + + Cl-

• The higher ammonium ion conc. causes the first equation to shift backwards; decreasing the conc. of hydroxide ions enough to exceed the Ksp of group III cations only