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Question

Consider the following statements. The correct statement is(are)

A
A monobasic acid with Ka=105 and a pH=5. The degree of dissociation of this acid is 50%.
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B
The Le-Chatelier's principle is not applicable to commom-ion effect.
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C
The pH of a mixture containing 400 mL of 0.1 M H2SO4 and 400 mL of 0.1 M NaOH will be approximately 1.3.
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D
Ionic product of water is temperature dependent.
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Solution

The correct option is D Ionic product of water is temperature dependent.
The explanation (I),
millimoles of H+=400×0.1×2=80
Millimoles of OH=400×0.1×2=40
(Limiting reagent)

Millimoles of H+ left =8040=40
[H+]=40400+400=40800=120 M

pH= log[H+]= log(120)
= log 1 + log 2 +log 10
= 0 + 0.301 + 1
1.30
Hence, the statement I is correct.

In statement II, the ionic product of H2O is temperature dependent.
Kw=[H+][OH]=1014(mol/L)2 at 25o C
With increase in temperature, dissociation of H2O units into H+ and OH ions will also increase. As a result, the value of ionic product, [H+]×[OH] will be increased.

Hence, the statement II is correct.

In statement III, for a weak monobasic acid HA,
HA(1α) C MHα C M++Aα C M
pH of the solution is,
[H+]=105 M=αC
Ka=αC×αC(1α)C=105×α1C
105=105×α1α
α=0.5
α%=50

In statement IV, Le- Chatelier's principle is applicable to common ion effect. Because, in presence of common ion (given) by strong electrolyte (say, Na+A), the product of the concentration terms in RHS increases. For the weaker electrolyte, HA (say) the equilibrium shifts to the LHS,
HAH++A

As a result dissociation of HA gets supressed. Hence, the statement IV is incorrect.

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