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1

AIPMT 2003

MCQ (Single Correct Answer)
Which one of the following statements is not true?
A
Among halide ions, iodide is the most powerful reducing agent.
B
Fluorine is the only halogen that does not show a variable oxidation state.
C
HOCl is a stronger acid than HOBr.
D
HF is a stronger acid than HCl.

Explanation

F is more electronegative than Cl therefore HF bond is stronger than HCl and hence proton is not given off easily and hence HF is a weakest acid.
2

AIPMT 2003

MCQ (Single Correct Answer)
The equilibrium constants of the following are

N2 + 3H2 $$\rightleftharpoons$$ 2NH3;     K1

N2 + O2 $$\rightleftharpoons$$ 2NO;     K2

H2 + $${1 \over 2}$$O2 $$\rightleftharpoons$$ H2O;     K3

The equilibrium constant (K) of the reaction :

2NH3 + $${5 \over 2}$$ O2 $$\rightleftharpoons$$ 2NO + 3H2O will be
A
K2K33/K1
B
K2K3/K1
C
K23K3/K1
D
K1K33/K2

Explanation

2NH3 $$\rightleftharpoons$$ N2 + 3H2;     $${1 \over {{K_1}}}$$

N2 + O2 $$\rightleftharpoons$$ 2NO;     K2

3H2 + $${3 \over 2}$$O2 $$\rightleftharpoons$$ 3H2O;     (K3)3

By adding all equations, we get

2NH3 + $${5 \over 2}$$ O2 $$\rightleftharpoons$$ 2NO + 3H2O

$$ \therefore $$ K = $${{{K_2} \times {{\left( {{K_3}} \right)}^3}} \over {{K_1}}}$$
3

AIPMT 2002

MCQ (Single Correct Answer)
Solution of 0.1 N NH4OH and 0.1 N NH4Cl has pH 9.25. Then find out pKb of NH4OH
A
9.25
B
4.75
C
3.75
D
8.25

Explanation

NH4OH and NH4Cl constitute to form a basic buffer.

pOH = pKb + log $${{\left[ {Salt} \right]} \over {\left[ {Base} \right]}}$$

We know, pOH+ pH = 14 or pOH = 14 – pH

$$ \therefore $$ 14 - pH - $$\log {{\left[ {Salt} \right]} \over {\left[ {Base} \right]}}$$ = pKb

$$ \Rightarrow $$ 14 - 9.25 - $$\log {{0.1} \over {0.1}}$$ = pKb

$$ \Rightarrow $$ 14 – 9.25 – 0 = pKb

$$ \Rightarrow $$ pKb = 4.75
4

AIPMT 2002

MCQ (Single Correct Answer)
Reaction BaO2(g) $$\rightleftharpoons$$ BaO(s) + O2(g); $$\Delta $$H = +ve. In equilibrium condition, pressure of O2 depends on
A
increase mass of BaO2
B
increase mass of BaO
C
increase temperature on equilibrium
D
increase mass of BaO2 and BaO both.

Explanation

For the reaction

BaO2(g) $$\rightleftharpoons$$ BaO(s) + O2(g); H = +ve.

At equilibrium Kp = PO2

[For solid and liquids concentration term is taken as unity]

Hence, the value of equilibrium constant depends only upon partial pressure of O2. Further on increasing temperature formation of O2 increases as this is an endothermic reaction.

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