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Equilibrium

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Question
CBSEENCH11008301
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A buffer solution is prepared in which the concentration of NH3 is 0.30 M and the concentration of NH4 is 0.20 M. If the equilibrium constant, Kb for NH3 equals 1.8 x 10-5, what is the pH of this solution?

log ( 2.7 = 0.43)

  • 9.43

  • 11.72

  • 8.73

  • 9.08

Solution

A.

9.43

pOH space equals space pK straight b space plus space log space fraction numerator left square bracket Salt right square bracket over denominator left square bracket Base right square bracket end fraction equals negative space log space straight K subscript straight b space plus space log space fraction numerator open square brackets Salt close square brackets over denominator left square bracket Base right square bracket end fraction equals negative space log space 1.8 space straight x space 10 to the power of negative 5 end exponent space plus space log space fraction numerator 0.20 over denominator 0.30 end fraction space equals space minus 5 minus 0.25 space plus space left parenthesis negative 0.176 space right parenthesis therefore comma 4.75 space minus space 0.176 space equals space 4.57 therefore comma space pH space equals space 14 space minus space 4.57 space equals space 9.43

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Question
CBSEENCH11008424
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A Weak acid, HA has a Ka of 1.00 x 10-5. If 0.100 mole of this percentage of acid dissociated at equilibrium is closest to:

  • 99.0%

  • 1.00%

  • 99.9 %

  • 0.100%

Solution

B.

1.00%

HA space leftwards harpoon over rightwards harpoon space straight H to the power of plus space plus space straight A to the power of minus straight K subscript straight a space space equals space fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket straight A to the power of minus right square bracket over denominator left square bracket HA right square bracket end fraction space equals space fraction numerator left square bracket straight H to the power of plus right square bracket over denominator left square bracket HA right square bracket end fraction left square bracket straight H to the power of plus right square bracket space equals space square root of straight K subscript straight a left square bracket HA right square bracket end root space equals space square root of 1 space straight x space 10 to the power of negative 5 end exponent space 0.1 end root space equals square root of 1 space straight x 10 to the power of negative 6 end exponent end root space equals space 1 space straight x space 10 to the power of negative 3 end exponent straight alpha space equals space fraction numerator actual space ionization over denominator molar space concentration end fraction space equals space fraction numerator 10 to the power of negative 3 end exponent over denominator 0.1 end fraction space equals space 10 to the power of negative 2 end exponent percent sign space of space acid space dissociated space space equals space 10 to the power of negative 2 end exponent space straight x space 100 space equals space 1 percent sign

Question
CBSEENCH11008334
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An aqueous solution of which of the following compounds is the best conductor of electric current?

  • Acetic acid C2H4O2

  • Hydrochloric acid, HCl

  • Ammonia, NH3

  • Fructose, C6H12O6

Solution

B.

Hydrochloric acid, HCl

HCl is strong acid and dissociates completely. Hence, it conducts electricity best in its aqueous solution.

Question
CBSEENCH11008408
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Base strength of 

1. space straight H subscript 3 straight C straight C with minus on top straight H subscript 2 2. space straight H subscript 2 straight C equals straight C with minus on top straight H 3. space straight H minus space straight C identical to straight C with minus on top

is in the order of 

  • (2) > (1) > (3)

  • (3) > (2) > (1) 

  • (1) > (3)> (2) 

  • (1) > (2) > (3)

Solution

D.

(1) > (2) > (3)

Stronger the acid, weaker is its conjugate base. 
The strength of their conjugate acids are in the order: 
HC space identical to CH space greater than thin space straight H subscript 2 straight C space equals CH subscript 2 space greater than thin space CH subscript 3 minus CH subscript 3
Therefore, the correct order of strength of their conjugate base is: 
CH subscript 3 CH subscript 2 to the power of minus greater than thin space straight H subscript 2 straight C space equals straight C to the power of minus straight H space greater than thin space HC identical to straight C to the power of minus

Question
CBSEENCH11008275
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Buffer solutions have constant acidity and alkalinity because

  • these give unionised acid or base on reaction with added acid or alkali

  • acids and alkalies in these solutions are shielded from attack by other ions.

  • they have a large excess of H+ or OH- ions

  • they have fixed value of pH

Solution

A.

these give unionised acid or base on reaction with added acid or alkali

If a small amount of an acid or alkali is added to a buffer solution, it converts them into unionised acid or base. Thus, remains unaffected or in other words its acidity/alkalinity remains constant. e.g.,
H3O+ + A-  ⇌ H2O +HA
-OH +HA → H2O +A-
If acid is added, it reacts with A- to form undissociated HA. Similarly, if base/alkali is added, O H- combines with HA to give H2O and A- and thus, maintains the acidity/alkalinity of buffer solution.

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