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Why is it necessary to do heating in analysis of cations?​

Sagot :

rozelcastaneda15

Answer:

Experiment 7: Qualitative Analysis of Cations 1

Experiment 7: Qualitative Analysis of Cations1

Purpose: Develop a systematic scheme of separation and analysis of a selected group of cations.

Introduction

In this experiment you will separate and identify the cations in an unknown mixture. The

possible ions are Ag+

, Cu2+, Fe3+, Cr3+, Zn2+, and Ba2+. For the separation and detection of the

cations, you will use the ability of these ions to form precipitates, to form complex ions, or to

show amphoteric behavior (act as either an acid or a base). In the first week you do preliminary

tests with each cation to discover each ion’s characteristic behavior. From the data obtained, you

devise a scheme for separating and identifying this group of cations. In the second week you

apply this scheme to the analysis of an unknown containing one or more of these cations.

I. Sparingly Soluble Salts

Some cations form relatively insoluble salts that precipitate out of aqueous solution. For

example, chloride salts are generally soluble with the notable exceptions of the salts containing

Ag+

, Pb+2, and Hg2

+2. Similarly, most sulfate salts are soluble with the exceptions of BaSO4,

PbSO4, Hg2SO4, and CaSO4. The formation of a precipitate upon introduction of chloride or

sulfate into solution is a positive test for Ag+

or Ba+2, respectively.

Many hydroxide salts are also quite insoluble. For example, the solubility of Fe+3 ions in basic

solution is governed by the reaction:

Fe(OH)3 (s) →← Fe+3 (aq) + 3 OH-

(aq) (1)

The corresponding equilibrium expression is:

Ksp = [Fe+3][OH-

]

3

(2)

The Ksp of 6.3x10-38 shows that Fe(OH)3 is a very sparingly soluble salt under most conditions.

Equation (2) can be solved for the solubility of Fe+3 ions in solution:

solubility = [Fe+3] =

Ksp

[OH-

]

3

(3)

Therefore, solubility rapidly decreases as the solution becomes more basic, Figure 1.

Figure 1: Solubility of Fe(OH)3 and Zn(OH)2 as a function of pH.

0.000

0.002

0.004

0.006

0.008

0.010

2 3 4 5 6 7 8

Solubility (M)

pH basic

soluble

insoluble

Fe(OH)3 Zn(OH)2

acetic acid + acetate

buffer

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