Chemistry NCERT Exemplar Solutions Class 11th Chapter Seven

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Ans: Let us assume S be the solubility of Al (OH)³.

K_sp= [Al³⁺] [OH^-]3= (S) (3S)³=27S⁴

S⁴=27K_sp=27 * 10²⁷ * 10^-11=1 * 10^-12

S=1*10⁻³ mol L^-1.

(i) Solubility of Al (OH)³ ? : Molar mass of Al (OH)³  is 78g. Hence, solubility of Al (OH)³ ? in gL^-1=1 * 10³ * 78gL^-1=78 * 10-3gL¹=7.8 * 10^-2gL^-1
(ii) pH of the solution: S=1* 10⁻³mol L^-1

OH=3S= 3*1* 10⁻³ = 3* 10⁻³

pOH=3−log³

pH=14−pOH= 11+log³= 11.4771.

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Ans: pH of HOCL = 2.85

But, - pH = log [H⁺]

 -2.85=log [H⁺]

= -3.15 = log [H⁺]

  [H⁺]=anti log3.15

  [H⁺]= 1.413* 10^-3 

or weak mono basic acid

[H⁺]= $\sqrt[]{K_a*}$  C

= $K_a$  = $\frac{{\left. [{\mathrm{H}}^{+}\right]}^2}{\mathrm{C}}$  = $\frac{{\left(\mathrm{}1.43*{10}^{-3}\right)}^2\mathrm{ }\mathrm{ }}{0.08}$

=24.957 $*\mathrm{}$ 10^-6

=2.4957 $*\mathrm{}$ 10^-5

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Ans: Given with the pH = 5 as, pH= - log  [ H⁺ ] 

  [ H⁺ ] =10^-5M

On 100 times dilution-

  [ H⁺ ] =10^-7M.

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Ans: Here concentration of water cannot be neglected since the solution is very dilute. pH will be less than 7.0. Hence, the total concentration is given as -

  [ H₃O⁺ ] =10^-8+10^-7M.

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Ans: At a given time the reaction quotient Q for the reaction will be given by the expression:

Q= $\frac{\left[\mathbf{H}2\right]\left[\mathbf{I}2\right]}{\left[\mathbf{H}\mathbf{I}\right]\mathrm{}}$  = $\frac{[1*10??*1*10??]}{{\left(2*\mathrm{}10^{-5}\right)}^2}$  = $\frac{1}{2}\mathrm{}$ = 0.25

 = 2.5*10⁻¹

As the value of the reaction quotient is greater than the value of K_c i.e. 1*10⁻⁴ the reaction will proceed in the reverse direction.

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Ans: Correct order of pH is -

NH₄Cl  C₆H₅COONH₄  KNO₃  CH₃COONa

Salts of strong acid and strong base do not hydrolyse and form neutral solution thus, pH will be nearly 7 of KNO₃?  

In sodium acetate, acetic acid remains unionised this results in increase in OH- concentration and pH will be more than 7. 

NH₄Cl formed from weak base,  NH₄OH and strong acid,  HCl, in water dissociates completely, aq. ammonium ions undergo hydrolysis with water to form NH₄OHand H₊ ions resulting in less pH value.

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Ans: Conjugate acid of a weak base is always stronger and conjugate base of weak acid is always strong.

The weak acids are:

⇒H₂O, ROH, CH₃COOH, HCl

⇒HCl > CH₃COOH > ROH > H₂O

⇒ Conjugate base order ⇒ OH⁻>RO⁻>CH₃COO⁻>Cl⁻

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Ans: Higher the value of Kb stronger will be the base. 

K_b-  5.4 * 10^-4 >1.77 * 10^-5 > 1.77 * 10^-9> 1.3 * 10^-14

Decreasing order of basic strength. 

Dimethylamine > Ammonia > Pyridine > Urea

Hence among the given bases, the strongest base is Dimethylamine.