QUANTITATIVE AND QUALITATIVE ANALYSIS Identification of ions

WEEK 2

TOPIC: QUANTITATIVE AND QUALITATIVE ANALYSIS

CONTENT:

Identification of ions
Test for gases

Identification of fats and oils

Sub-topic 1: IDENTIFICATION OF IONS

IDENTIFICATION OF CATIONS AND ANIONS

CATIONS: Zn²⁺, Al³⁺, pb²⁺ , Ca²⁺ ,Fe²⁺Fe³⁺, Cu²⁺, NH₄⁺

THE USE OF NaOH_(aq): If a few drops of NaoH_(aq) is added to aqueous solutions of the salts of the above cations the following observations will be made. If it is:

a. Zn²⁺: There will be formation of a white gelatinous precipitate which will be soluble in excess NaoH_(aq).

b. Al³⁺There will be formation of a white gelatinous precipitate which will be soluble in excess NaoH _(aq).

c. Pb²⁺:There will be formation of a white precipitate which will be soluble in excess NaoH_(aq).

d. Ca²⁺: There will be formation of a white precipitate which will be soluble in excess NaoH_(aq).

e. Fe²⁺: There will be formation of a green gelatinous precipitate which will be insoluble in excess NaOH_(aq).

f. Fe³⁺There will be formation of reddish- brown gelatinous precipitate which will be insoluble in excess NaoH_(aq).

g. Cu^2+: There will be a formation of a blue precipitate which is insoluble in excess NaoH_(aq).

h. NH₄⁺_:If NaoH_(aq) is added to the test solution and warmed, a colourless gas with irritating smell which will turn moist red litmus paper to blue will be evolved. Thus, showing that the test solution is containing NH₄⁺ i. e. the test solution is an NH₄⁺ salt solution.

ANIONS: Cl^–, NO₃^–, SO₄^2-, SO₃^2-, CO₃^2-:

The following tests help to identify these anions:

Cl^– : To the test solution add dilute HNO₃ acid (there will not be visible reaction), followed by AgNO_3(aq), (there will be formation of a white precipitate). To the precipitate add NH_3(aq) in excess and shake . If the precipitate dissolves then the test solution is containing Cl^– ions.
NO₃^–: To the test solution add FRESHLY prepared FeSO₄ solution followed by concentrated H₂SO₄ acid. Formation of a brown ring indicates the presence of NO₃^– ions.
SO₄^2- : To the test solution add dilute HCI acid,(there will not be any visible reaction, this is just to get rid of any other ions that may tamper with the experiment), followed by BaCl₂(aq). There will be formation of a white precipitate which will be insoluble in excess dil.HCI. This shows that SO₄^2- is present in the test solution.

SO₃^2-: To the test solution add BaCl_2(aq). There will be formation of a white precipitate which will be soluble in dil. HCl acid
CO₃^2-: The test for CO₃^2-, is also carried out exactly in the same way as in (d) above. The same result is obtained. To differentiate between SO₃^2- and CO₃^2- , add MgSO_4(aq) solution to solutions containing SO₃^2-and CO₃^2- ions in different test tubes. The one that gives a white precipitate is CO₃^2-. The example below shows how to report salt analysis.

Worked Example

Credit will be given for strict adherence to instructions, for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in your work book, in ink, at the time they are made. C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas (es) evolved. State the conclusion you draw from the result of each test. (a) Put all of C in a test tube and add about 10cm³ of distilled water. Shake thoroughly and filter. Keep both the filtrate and the residue. Divide the filtrate into three portions. (i) To first portion, add NaOH_(aq) in drops and then in excess. (ii) To the second portion, add NH₃solution in drops and then in excess. (iii) To the third portion, BaCl_2(aq) followed by dilute HCl. (b) Divide the residue into two portions. (i) Heat the first portion strongly in a test tube. (ii) Add dilute HCl to the second portion.

Solution

TEST	OBSERVATION	INFERENCE
(a)(i) C + H₂O, then filter	Green residue and blue filtrate were observed	C is a mixture of soluble and insoluble salts.
(ii) Filtrate + NaOH_(aq) in drops, then in excess	A blue gelatinous precipitate which is insoluble in excess was formed.	Cu²⁺ present
(iii) Filtrate + NH_3(aq) in drops, then in excess	A pale blue gelatinous precipitate was formed. The precipitate dissolves or is soluble in excess NH_3(aq) to give deep blue solution.	Cu²⁺ is confirmed
(iv) Filtrate + BaCl₂ + dil. HCl	A white precipitate, insoluble in dil. HCl was observed.	SO₄^2- is present
(b)(i) First portion of residue + heat	The colour changed from green to black powder
(ii) Second portion of residue + dil. HCl	There was effervescence and a colourless, odourless gas which turn lime water milky was given off	Gas is CO₂ from CO₃^2-

EVALUATION

Credit will be given for strict adherence to instructions, for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in your work book, in ink, at the time they are made. C is a mixture of two salts. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion you draw from the result of each test. (a) Put C into a beaker and add about 10cm³of distilled water, stir the mixture and filter. Test the filtrate with litmus papers. Keep the residue and filtrate. (b)(i) To about 2cm³ of the filtrate, add few drops of aqueous HNO₃ followed by AgNO_3(aq) .

(ii) Add excess NH₃ solution to the resulting mixture. (c) To about half of the residue from 1(a) above, add about 5cm³ of dilute HNO₃ in drops. Divide the resulting solution into two equal portions. (d) (i) To the first portion, add ammonia solution in drops and then in excess. (ii) To the second portion, add dilute HCl.

Sub-topic 2: Test for gases

SO₂, H₂S, NH₃, CO₂, NO₂, HCI, O₂, CI₂ and H₂

GAS	ODOUR	COLOUR		COMFIRMATORY TEST	SUSPECTED SALT
SO₂	Irritating smell		Colourless	Changes acidified k₂cr₂0₇solution from orange to green. Change acidified KMNO₄ solution from purple to colourless. It turns moist blue litmus paper to red.	SO₃^2- of Na or K
H₂S	Rotten egg smell		Colourless	It changes acidified K₂Cr₂O₇ solution from orange to green with yellow sulphur deposit. It changes acidified KMnO₄Solution from purple to colourless with yellow sulphur deposits Turns (CH₃ COO₂)Pb solution or paper black. Turns Pb(NO₃)₂ paper or solution to black. Turns moist blue litmus paper red.	S^2- of metal e.g. FeS
NH₃	Choking smell		Colourless	Turn moist red litmus paper to blue. Gives white fumes with HCI	NH₄⁺ salts
CO₂	Odourless		Colourless	Turns moist blue litmus paper red. Turns lime water milky	HCO₃^–and CO₃^2- Salts
NO₂	Irritating smell		Reddish-brown	It turns moist blue litmus paper to red. Turns moist starch-iodide paper to blue-black	NO₃^– of heavy metals e.g.Pb(NO₃)₂
HCI	Sharp irritating smell		Colourless	Turns moist blue litmus paper red. Form white fumes with ammonia gas.	CI^– salts
O₂	Odourless		Colourless	Rekindle a glowing splint	KClO₃
CI₂	Pungent smell		Greenish- yellow	Turns moist blue litmus paper to pink then bleaches it. Turns moist starch – iodide paper to dark blue.	Cl^– of Na, Mg
H2	Odourless		Colourless	Gives a pop sound when ignited in the air.	H2 is obtained when certain metals e.g. Zn, Sn and Al react with acids, water, steam or hot concentrated NaOH or KOH Solution.

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Tags:Chemistry, SECOND TERM, SS 3

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