Part 15 Analytical Chemistry

Question 1

Suggest a test for H2 (g).

Answer 1

It burns with pop sound.

Question 2

Suggest a test for O2 (g).

Answer 2

It relights glowing splint.

Question 3

Suggest a chemical test for CO2 (g).

Answer 3

It turns limewater milky. Then the milky solution becomes colourless.

Question 4

Suggest a chemical test for Cl2 (g).

Answer 4

It turns moist blue litmus paper red then bleaches it very quickly.

Question 5

Suggest a chemical test for NH3 (g)

Answer 5

It turns moist litmus paper blue // bring the gas close to an unstoppered bottle of conc. HCl. Dense white fumes formed.

Question 6

Suggest a chemical test for HCl (g).

Answer 6

Bring the gas close to an unstoppered bottle of conc. NH3(aq). Dense white fumes formed.

Question 7

Suggest a chemical test for SO2 (g).

Answer 7

It turns limewater milky // it turns a piece of filter paper soaked with acidified K2Cr2O7 solution from orange to green // it turns brown bromine water colourless // it decolourises petals temporarily // pass the gas to (Cr2O7)2- / H+ (or (MnO4)- / H+. The solution turns from orange to green.

Question 8

Suggest a test for H2O (g).

Answer 8

It turns dry cobalt(II) chloride paper from blue to pink. // it turns white anhydrous copper(II) sulphate powder blue.

Question 9

Suggest a test for Ca2+ ions.

Answer 9

Flame test, It burns with brick-red flame.

Question 10

Suggest a test for Cu2+ ions.

Answer 10

It burns with bluish-green flame.

Question 11

Suggest a test for K+ ions.

Answer 11

It burns with lilac flames.

Question 12

Suggest a test for Na+ ions.

Answer 12

It burns with golden yellow flame.

Question 13

Suggest a chemical test for Cl- ions.

Answer 13

Add acidified silver nitrate solution.
White precipitate formed.

Question 14

Suggest a chemical test for Br- ions.

Answer 14

Add acidified silver nitrate solution.
Pale yellow precipitate formed.

Question 15

Suggest a chemical test for OCl- ions in aqueous solution.

Answer 15

Add a few drops of HCl (aq).
Pale yellowish-green gas evolved which turns moist blue litmus paper red then bleaches it.

Question 16

Suggest a chemical test for (SO3)2- ions

Answer 16

Add Br2(aq), the solutions changes from brown to colourless. // Add (Cr2O7)2- / H+ , the solution changes from orange to green. // Add MnO4- / H+ , the solution changes from purple colourless.

Question 17

suggest a chemical test for alkene. (C=C)

Answer 17

add Br(in organic solvent). The bromine solution changes from orange to colourless rapidly. // add (Cr2O7)2- / H+. The solution changes from purple to colourless.

Question 18

suggest a chemical test for hydroxyl group. (–OH)

Answer 18

Warm with (Cr2O7)2- / H+. The solution changes from orange to green for 1° and 2° alcohol.

Question 19

Suggest a chemical test for alkene.( >C=C< )

Answer 19

Add Br(in organic solvent). The bromine solution changes from orange to colourless rapidly. // Add MnO4- / H+. The solution changes from purple to colourless.

Question 20

Suggest a chemical test for hydroxyl group.( –OH )

Answer 20

Warm with (Cr2O7)2- / H+. The solution changes from orange to green for 1° and 2° alcohol. // Heat with a carboxylic acid in the presence of concentrated sulphuric acid. An ester with a characteristic pleasant fruity smell forms.

Question 21

Suggest a chemical test for aldehydes and ketone.( >C=O )

Answer 21

Add 2-4-dinitrophenylhydrazine solutions. Red, yellow or orange precipitate formed

Question 22

suggest a chemical test for aldehydes.( –CHO )

Answer 22

Warm with Tollens’ reagent. A silver mirror forms on the inner wall of the test tube. // Add acidified K2Cr2O7 / H+. The solution turns from orange to green.

Question 23

Suggest a chemical test for Carbonyl group.( –COOH )

Answer 23

Heat with an alcohol in the presence of concentrated sulphuric acid. An ester with a characteristic pleasant fruity smell forms.

Question 24

What is meant by the term “Rf value” ?

Answer 24

Rf value of a substance is the ratio between the migration distance of the substance and the migration distance of the solvent front during chromatography.

Question 25

Suggest one method that can be used to make the two spots in the chromatographic plate become visible.

Answer 25

Put the chromatographic plate into a jar that is saturated with iodine vapur. The spots will appear brown. // Irradiate the plate with UV. The stationary phase is flourescent while the two spots are not.

Question 26

Explain briefly why chromatography can be used to seperate a mixture

Answer 26

Different substance have different adsorptivity to the stationary phase and different solubility in mobile phase.

Question 27

Suggest how you can identify that the chemical you obtained is pure.

Answer 27

Run a TLC / paper chromatography for the sample; if only one spot is detected on the chromatogram, the chemcial obtained is pure // (if the m.p / b.p. is given)Measure the m.p. / b.p. of the chemical obtained if the m.p. / b.p. is (...) it may be pure. //

Question 28

Explain why filtration can be used to remove mud particals from muddy water, but cannot be used to remove sodium chloride from sea water.

Answer 28

Sea water is solution while muddy water is suspension. The size of ions in water is much smaller than that of the mud particals in muddy water. Ions in sea water can pass through pores in filter device while mud particals cannot.

Question 29

Suggest one property of solvents rendering them to be used in the liquid-liquid extraction.

Answer 29

The solvents are immiscible with water // The solvents do not react with the organic compounds // The solvents can dissolve more oragnic compounds than water can // The solvents have low boiling points / can evaporate easily

Question 30

Why are standard iodine solutions not prepared directly from solid iodine ?

Answer 30

I2(s) is very volatile. It is difficult to weigh accurately a sample of I2 // I2 reaxts readily with reducing agents in the envirnent. I2(s) in laboratory usually contains impurities

Question 31

Why is SO2 is added to white wine ?

Answer 31

SO2 acts as an antioxidant in wine. It helps prevent the oxidation of ethanol to ethanoic acid by the actions of oxygen and bateria in air

Question 32

State one condition that a reaction must fulfill in order that it can be used in volumetric analysis.

Answer 32

A significant observable change can be easily be detected with the complete consumption of the reagent in the conical flask, i.e. availability of suitable indicator // The reaction must be significantly complete // The rate of reaction must be fast enough to be practical

Question 33

What is meant by the term "primary standard" ?

Answer 33

A substance which can be used to prepare standard solution by dissolving a known mass of the substance in a solvent and making up the solution to a known volume

Question 34

Explain why NaOH / KOH cannot be used as primary standard.

Answer 34

NaOH / KOH can absorb water and react with CO2 in air. Mass of NaOH cannot be accurately measured