P Block Elements(12th Grade > Chemistry ) Questions and answers for exam Preparation

Question 1. A white coloured inorganic salt gives the following reactions.
(i) It is soluble in water and the solution has sweet taste.
The solution turns black in presence of

H_{2} S

ii) The solution gives a white precipitate with dilute HCl which is soluble in hot water.
iii) The salt when heated gives acetone and a yellow coloured residue. Identify the salt (A) and give chemical reactions.

NaCl
Sn(HCOO)2
Pb(CH3COO)2
CaCO3

Explanation:-

Answer: Option C. -> Pb(CH3COO)2
:
C

(A) i s P b (C H_{3} C O O)_{2} P b (C H_{3} C O O)_{2} + H_{2} S \to P b S + 2 C H_{3} C O O H Black P b (C H_{3} C O O)_{2} + 2 H C l \to P b C l_{2} + 2 C H_{3} C O O H Soluble in hot water P b (C H_{3} C O O)_{2} \to P b O + C O_{2} + C H_{3} C O C H_{3}]

Question 2. Which of the following mineral does not contain Al ?

Cryolite
Mica
Felspar
Fluorspar

Explanation:-

Answer: Option D. -> Fluorspar
:
D
Fluorspar ;

C a F_{2}

Question 3. In the extraction of aluminium, the function of cryolite is to:

Lower the melting point of alumina
Increase the melting point of alumina
Remove impurities from alumina
Minimise the anodic effect

Explanation:-

Answer: Option A. -> Lower the melting point of alumina
:
A
Melting point of

A I_{2} O_{3}

is very high, adding impurities reduces melting point

Question 4.

C (O H)_{4}

is unstable because a carbon atom cannot hold more than one -OH groups but

S i (O H)_{4}

is a stable compound because

C-O bond energy is low
C-O bond energy is high
Si-O bond energy is low
Si-O bond length is small

Explanation:-

Answer: Option A. -> C-O bond energy is low
:
A
C - O bond energy is 358 kj .

m o l^{- 1}

Si - O bond energy is368 kj .

m o l^{- 1}

Question 5. An inorganic compound (A) is a strong reducing agent. Its hydrolysis in water gives a white turbidity (B). Aqueous solution of (A) gives a white precipitate (C) with NaOH solution which is soluble in excess of NaOH. (A) reduces auric chloride to produce purple of cassius. (A) also reduces iodine and gives chromyl chloride test. What is 'B' in the chemical reactions.

SnCl2
Sn(OH)Cl
SnCl4
Na2SnO2

Explanation:-

Answer: Option B. -> Sn(OH)Cl
:
B

(A) = S n C l_{2}; (B) = S n (O H) C l : (C) = S n (O H)_{2} S n C l_{2} + H_{2} O \to S n (O H) C l + H C l (A) (B) S n C l_{2} + N a O H \to S n (O H)_{2} + 2 N a C l, S n (O H)_{2} + 2 N a O H \to N a_{2} S n O_{2} + 2 H_{2} O 3 S n C l_{2} + 2 A u C l_{3} \to 3 S n C l_{4} + 2 A u S n C l_{2} + 2 H C l + l_{2} \to S n C l_{4} + 2 H l

Question 6. The complete hydrolysis of

(C H_{3})_{2} S i C l_{2}

forms-

(CH3)2SiO
(CH3)2Si(OH)2
CH3 CH3 | |CH3−Si−O−Si−CH3 | | CH3 CH3

Explanation:-

Answer: Option C. -> Sn(OH)Cl
:
C

(C H_{3})_{2} S i (O H)_{2}

formed will lose a water molecule to form long chains.

Question 7. Identify (X) to (Z) in the following sequence of reactions -

C a O + C 2000^{\circ} C - --- \to (X) + (Y) 1100^{\circ} C ↓ + N_{2} (z) h y d r o l y s i s - ----- \to (P) gas

(X)≡CaC2(Y)≡CO(Z)≡Ca3N2(P)≡NH3
(X)≡CaC2(Y)≡C(Z)≡Ca(NH2)2(P)≡NH3
(X)≡CaC2(Y)≡C(Z)≡CaCN2(P)≡NH3
(X)≡CaC2(Y)≡CO(Z)≡CaCN2(P)≡NH3

Explanation:-

Answer: Option D. -> (X)≡CaC2(Y)≡CO(Z)≡CaCN2(P)≡NH3
:
D
X =

C a C_{2}

, heating calcium carbide with

N_{2}

gives calcium cyanamide

(C a C N_{2})

, which in turn liberates

N H_{3}

on hydrolysis

Question 8. Thallium shows different oxidation states because -

It is a transition metal
Of inert pair effect
Of its amphoteric character
Of its high reactivity

Explanation:-

Answer: Option B. -> Of inert pair effect
:
B
Inert pair effect is the phenomenon in which outer shell (ns2)electrons penetrate to
(n - 1) d electrons and thus becomes closer to nucleus and are more effectively pulled towards nucleus. This results in less availability of ns electrons for bonding. The inert pair effects begins when n =4 and increases with increasing value of n.

Question 9. Aluminium vessels should not be washed with materials containing washing soda because:

Washing soda is expensive
Washing soda is easily decomposed
Washing soda reacts with aluminium to form soluble aluminate
Washing soda reacts with aluminium to from insoluble aluminium oxide

Explanation:-

Answer: Option C. -> Washing soda reacts with aluminium to form soluble aluminate
:
C

2 N a O H + 2 A l + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}

Question 10. An aqueous solution of potash alum is -

Acidic
Alkaline
Neutral
None of these

Explanation:-

Answer: Option A. -> Acidic
:
A

K_{2} S O_{4} . A I_{2} (S O_{4})_{3} . 24 H_{2} O K_{2} S O_{4} + H_{2} O \to K (O H) + H_{2} S O_{4} A I_{2} (S O_{4})_{3} + H_{2} O \to A l (O H)_{3} + H_{2} S O_{4}