Qualitative Analysis Test

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Qualitative Analysis Test - 3

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Weekly Quiz Competition

Question 1
1 / -0

Consider following compounds and decide as to which of the following statements are true ?

A
(II) gives no reaction with Na metal, however, 1 mole of (IV) on reaction with Na metal will liberate 22.4 litres of H₂ gas at STP

B
(I) will give brisk effervescence on addition of NaHCO₃ but will not bring any change in the colour of Br₂ water

C
(III) Iiberates H2 gas with Na metal, gives white precipitate with Tollen's reagent but does not respond towards lucas reagent or 2, 4–DNP test.

D
(IV) gives turbidity with anhydrous ZnCI₂

Solution
Question 2
1 / -0

The Prussian blue colour obtained during the test of nitrogen by Lassaigne's test is due to the formation of-

A
Fe₄[Fe(CN)₆]₃

B
Na₃[Fe(CN)₆]

C
Fe(CN)₃

D
Na₄[Fe(CN)₅NOS]

Solution

Hint: Ferriferrocyanide is obtained as product.

The reaction takes place in Lassaigne's test are as follows:
Question 3
1 / -0

A compound that does not give a positive test in Lassaigne’s test for nitrogen is:-

A
Urea

B
Hydrazine

C
Azobenzene

D
Phenylhydrazine

Solution

Hint: Molecule must contain C-N bond

When a compound containing CN bond reacts with sodium metal then it will form NaCN as a product. If the molecule does not contain a C-N bond then NaCN is not possible.

The structures of given compounds are as follows:

So, Hydrazine H₂N-NH₂ does not contain a C-N bond, hence, it will not give Lassaigne’s test.
Question 4
1 / -0

The silver sulphate solution is used to separate:

A
nitrate and bromide

B
nitrate and chlorate

C
bromide and iodide

D
nitrate and nitrite

Solution
Question 5
1 / -0

Soda extract is prepared by-

A
Fusing soda and mixture, and then extracted with water

B
Dissolving NaHCO₃, and mixture in dil. HCl

C
Boiling Na₂CO₃, and mixture in dil.HCl

D
Boiling Na₂CO₃, and mixture in distilled water

Solution

Hint: The process to prepare soda extract is as follows:

To prepare soda extract, mixture/salt is mixed with pure Na₂CO₃ in 1 : 2 ratio and a sufficient amount of water are added. The whole content is boiled for 15-20 minutes and then filtered. The filtrate is used as soda extract.

The reaction is as follows:

(a) Some cations interfere in the confirmatory test of some anions when present in solution. Such cations are eliminated in the form of insoluble carbonates as a result of treatment with sodium carbonate.

(b) It becomes easier to identify anions in such mixture which are insoluble in water and dil. acids.
Question 6
1 / -0

The Lassaigne's extract is boiled with a concentration HNO₃ while testing for halogens. By doing so it

A
Helps in the precipitation of AgCl

B
Increases the solubility product of AgCl

C
Increases the concentration of NO₃^- ions

D
Decomposes Na₂S and NaCN, if formed.

Solution

Hint: H₂S is gas and HCN boiling point is 26 ^oC

Actually, Na₂S& NaCN is removed as H₂S & HCN by conc. HNO₃. Concentrate HNO₃ convert Na₂S into H₂S and NaCN into HCN. The reaction is as follows:

HCN is having boiling point 26 ^oC and H₂S is gas. So, they were removed easily.
Question 7
1 / -0

Lassaigne's test can detect -

A
Nitrogen, Sulphur, Halogens

B
Nitrogen, Cyanides, Suphur

C
Sodium, Sulphur, Halogens, Phosphorus

D
Nitrogen, Sulphur, Halogens, Phosphorus

Solution

HINT- This test is employed to detect the presence of nitrogen, sulphur, halogens, and phosphorous in an organic compound.

STEP1- These elements are present in the covalent form in an organic compound. These are converted into the ionic form by fusing the compound with sodium metal.

STEP2- The cyanide, sulphide, and halide of sodium formed are extracted from the fused mass by boiling it in distilled water. The extract so obtained is called Lassaigne’s extract. This Lassaigne’s extract is then tested for the presence of nitrogen, sulphur, halogens, and phosphorous.
Question 8
1 / -0

Nitric acid is added to sodium extract before adding silver nitrate for testing halogens because

A
Nitric acid reduces suphide

B
Nitric acid decomposes NaCN and Na₂S

C
Nitric acid oxidises the organic compound

D
Nitric acid acts as a dehydrating agent

Solution

Hint: Lassaigne's test is used to detect halogens, N, P, and S

Step 1:

While testing Lassaigne’s extract for the presence of halogens, it is first boiled with dilute nitric acid. This is done to decompose NaCN to HCN and Na₂S to H₂S and to expel these gases. That is, if any nitrogen and sulphur are present in the form of NaCN and Na₂S, then they are removed.

Step 2:

The chemical equations involved in the reaction are represented as:
Question 9
1 / -0

A mixture of calcium sulphate and camphor can be separated by-

A
Filtration

B
Evaporation

C
Sublimation

D
Chromatography

Solution

Hint: The process of sublimation is used to separate sublimating substances from non-sublimating compounds

In this process, the sublimable compound changes from solid to vapour state without passing through the liquid state. Camphor is a sublimable compound and calcium sulphate is a non-sublimable solid. Hence, on heating, camphor will sublime while calcium sulphate will be left behind.
Question 10
1 / -0

A gas among the following that can be absorbed by potassium hydroxide is -

A
Carbon dioxide

B
Silicon dioxide

C
Hydrogen

D
Carbon monoxide

Solution

Hint: Carbon dioxide is acidic in nature and potassium hydroxide is a strong base.

Step 1:

Carbon dioxide reacts with potassium hydroxide to form potassium carbonate and water. The reaction is as follows:

2KOH + CO₂ → K₂CO₃ + H₂O

Step 2:

The mass of the U-tube containing KOH increases. This increase in the mass of Utube gives the mass of CO₂ produced. From its mass, the percentage of carbon in the organic compound can be estimated.