Ionic Equilibrium | JEE Main Chemistry Chapter Wise Question Papers

Q 1 :
$K_{a}$ for ${CH}_{3} COOH$ is $1.8 \times 10^{- 5}$ and $K_{b}$ for ${NH}_{4} OH$ is $1.8 \times 10^{- 5} .$ The pH of ammonium acetate solution will be _______ . [2024]

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$As K_{a} of {CH}_{3} COOH = K_{b} of {NH}_{4} OH$

$So, {pK}_{a} of {CH}_{3} COOH = {pK}_{b} of {NH}_{4} OH$

$And p K_{a} - p K_{b} = 0$

For salt of weak acid and weak base:

$pH = \frac{1}{2} (p K_{w} + p K_{a} - p K_{b})$

$pH = \frac{1}{2} (14 + 0) = 7$

Q 2 :
pH of water is 7 at 25°C. If water is heated to 80°C, its pH will : [2025]

Remains the same
Decrease
Increase
$H^{+}$ concentration increases, ${OH}^{-}$ concentration decreases

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$H_{2} O ⇌ H^{+} + {OH}^{-}$

With increase in temperature, dissociation of water increases, hence $[H^{+}]$ increases. Increase of $[H^{+}]$ means decrease of pH $(p H = - \log [H^{+}]) .$

Q 3 :
An aqueous solution of HCl with pH 1.0 is diluted by adding equal volume of water (ignoring dissociation of water). The pH of HCl solution would

(Given log 2 = 0.30) [2025]

reduce to 0.5
increase to 1.3
remain same
increase to 2

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2

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(2)

Initial concentration of $H^{+} ({[H^{+}]}_{initial}) = 10^{- {pH}_{initial}} = 10^{- 1} M$

As equal volume is added, concentration of $H^{+}$ becomes half

$({[H^{+}]}_{initial}) = \frac{10^{- 1}}{2} M$

${pH}_{final} = - \log {[H^{+}]}_{final} = - \log (\frac{10^{- 1}}{2})$

$= - \log (10^{- 1}) + \log 2 = 1 + 0.3 = 1.3$

Q 4 :
A weak acid HA has degree of dissociation $x$ . Which option gives the correct expression of $(pH - p K_{a})$ ? [2025]

$\log (1 + 2 x)$
$0$
$\log (\frac{x}{1 - x})$
$\log (\frac{1 - x}{x})$

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(3)

$\begin{matrix} HA & ⇌ & H^{+} & + & A^{-} \\ (t=0) (moles) & a_{0} & 0 & 0 \\ (t=∞) (moles) & a (1 - x) & a_{0} x & a_{0} x \end{matrix}$

$K_{a} = \frac{[H^{+}] [A^{-}]}{[HA]} = \frac{[H^{+}] \times a_{0} x}{a_{0} (1 - x)}$

$K_{a} = \frac{[H^{+}] \times x}{(1 - x)}$

Taking $- \log$ on both sides

$- \log K_{a} = - \log (\frac{[H^{+}] \times x}{(1 - x)})$

$- \log K_{a} = - \log [H^{+}] - \log (\frac{x}{1 - x})$

$p K_{a} = pH - \log (\frac{x}{1 - x})$

$pH - p K_{a} = \log (\frac{x}{1 - x})$

Q 5 :
If 1 mM solution of ethylamine produces pH = 9, then the ionization constant $(K_{b})$ of ethylamine is $10^{- x}$ . The value of $x$ is ______ (nearest integer).

[The degree of ionization of ethylamine can be neglected with respect to unity.] [2025]

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$\begin{matrix} C_{2} H_{5} {NH}_{2} (a q) & + & H_{2} O & ⇌ & C_{2} H_{5} {NH}_{3}^{+} (a q) & + & O H^{-} (a q) \\ t = 0 & 10^{- 3} M & excess & 0 & 0 \\ t = \infty & 10^{- 3} (1 - α) & 10^{- 3} α M & 10^{- 3} α M \\ ≃ 10^{- 3} M \end{matrix}$

$pOH = 14 - pH = 14 - 9 = 5$

Thus ${[O H]}^{-} = 10^{- 5} M$

Also, $[C_{2} H_{5} {NH}_{3}^{+}] = {[O H]}^{-} = 10^{- 5} M$

$K_{b} = \frac{{[O H]}^{-} [C_{2} H_{5} {NH}_{3}^{+}]}{[C_{2} H_{5} {NH}_{2}]} = \frac{10^{- 5} \times 10^{- 5}}{10^{- 3}} = 10^{- 7}$

Q 6 :
The pH of a 0.01 M weak acid HX $(K_{a} = 4 \times 10^{- 10})$ is found to be 5. Now the acid solution is diluted with excess of water so that the pH of the solution changes to 6. The new concentration of the diluted weak acid is given as $x \times 10^{- 4}$ M. The value of $x$ is ______ (nearest integer). [2025]

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New concentration of $H^{+} ({[H^{+}]}_{new}) = 10^{-pH(new)} M = 10^{- 6} M$

$\sqrt{K_{a} C_{new}} = {[H^{+}]}_{new}$

$\sqrt{4 \times 10^{- 10} C_{new}} = 10^{- 6}$

$4 \times 10^{- 10} C_{new} = 10^{- 12}$

$C_{new} = 25 \times 10^{- 4} M$

Note: In the question it should be 0.25 M weak acid instead of 0.01 M weak acid. However, this data is not needed to solve the question.

Q 7 :
$x$ mg of $M g {(O H)}_{2}$ (molar mass = 58) is required to be dissolved in 1.0 L of water to produce a pH of 10.0 at 298 K. The value of $x$ is ____ mg. (Nearest integer)

(Given: $M g {(O H)}_{2}$ is assumed to dissociate completely in $H_{2} O$ ) [2025]

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$pOH = 14 - pH = 14 - 10 = 4$

$[O H^{-}] = 10^{- pOH} = 10^{- 4} M$

$Moles of hydroxide (n_{O H^{-}}) = [O H^{-}] \times volume$

$= 10^{- 4} M \times 1 L = 10^{- 4} mol$

$Moles of Mg(OH)_{2} (n_{Mg(OH)_{2}}) = \frac{n_{O H^{-}}}{2} = \frac{10^{- 4}}{2} mol$

$Mass of Mg(OH)_{2} = n_{Mg(OH)_{2}} \times M_{Mg(OH)_{2}}$

$= \frac{10^{- 4}}{2} \times 58 = 29 \times 10^{- 4} g = 2.9 mg ≈3 g$

Topic Question Set

Q 1 :
$K_{a}$ for ${CH}_{3} COOH$ is $1.8 \times 10^{- 5}$ and $K_{b}$ for ${NH}_{4} OH$ is $1.8 \times 10^{- 5} .$ The pH of ammonium acetate solution will be _______ . [2024]

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Q 2 :
pH of water is 7 at 25°C. If water is heated to 80°C, its pH will : [2025]

Q 3 :
An aqueous solution of HCl with pH 1.0 is diluted by adding equal volume of water (ignoring dissociation of water). The pH of HCl solution would

(Given log 2 = 0.30) [2025]

Q 4 :
A weak acid HA has degree of dissociation $x$ . Which option gives the correct expression of $(pH - p K_{a})$ ? [2025]

Q 5 :
If 1 mM solution of ethylamine produces pH = 9, then the ionization constant $(K_{b})$ of ethylamine is $10^{- x}$ . The value of $x$ is ______ (nearest integer).

[The degree of ionization of ethylamine can be neglected with respect to unity.] [2025]

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Q 7 :
$x$ mg of $M g {(O H)}_{2}$ (molar mass = 58) is required to be dissolved in 1.0 L of water to produce a pH of 10.0 at 298 K. The value of $x$ is ____ mg. (Nearest integer)

(Given: $M g {(O H)}_{2}$ is assumed to dissociate completely in $H_{2} O$ ) [2025]

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Topic Question Set

Q 1 : Ka for CH3COOH is 1.8×10-5 and Kb for NH4OH is 1.8×10-5. The pH of ammonium acetate solution will be _______ . [2024]

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Q 2 : pH of water is 7 at 25°C. If water is heated to 80°C, its pH will : [2025]

Q 3 : An aqueous solution of HCl with pH 1.0 is diluted by adding equal volume of water (ignoring dissociation of water). The pH of HCl solution would (Given log 2 = 0.30) [2025]

Q 4 : A weak acid HA has degree of dissociation x. Which option gives the correct expression of (pH-pKa)? [2025]

Q 5 : If 1 mM solution of ethylamine produces pH = 9, then the ionization constant (Kb) of ethylamine is 10-x. The value of x is ______ (nearest integer). [The degree of ionization of ethylamine can be neglected with respect to unity.] [2025]

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Q 6 : The pH of a 0.01 M weak acid HX (Ka=4×10-10) is found to be 5. Now the acid solution is diluted with excess of water so that the pH of the solution changes to 6. The new concentration of the diluted weak acid is given as x×10-4 M. The value of x is ______ (nearest integer). [2025]

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Q 7 : x mg of Mg(OH)2 (molar mass = 58) is required to be dissolved in 1.0 L of water to produce a pH of 10.0 at 298 K. The value of x is ____ mg. (Nearest integer) (Given: Mg(OH)2 is assumed to dissociate completely in H2O) [2025]

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Q 1 :
$K_{a}$ for ${CH}_{3} COOH$ is $1.8 \times 10^{- 5}$ and $K_{b}$ for ${NH}_{4} OH$ is $1.8 \times 10^{- 5} .$ The pH of ammonium acetate solution will be _______ . [2024]

Q 2 :
pH of water is 7 at 25°C. If water is heated to 80°C, its pH will : [2025]

Q 3 :
An aqueous solution of HCl with pH 1.0 is diluted by adding equal volume of water (ignoring dissociation of water). The pH of HCl solution would

(Given log 2 = 0.30) [2025]

Q 4 :
A weak acid HA has degree of dissociation $x$ . Which option gives the correct expression of $(pH - p K_{a})$ ? [2025]

Q 5 :
If 1 mM solution of ethylamine produces pH = 9, then the ionization constant $(K_{b})$ of ethylamine is $10^{- x}$ . The value of $x$ is ______ (nearest integer).

[The degree of ionization of ethylamine can be neglected with respect to unity.] [2025]

Q 7 :
$x$ mg of $M g {(O H)}_{2}$ (molar mass = 58) is required to be dissolved in 1.0 L of water to produce a pH of 10.0 at 298 K. The value of $x$ is ____ mg. (Nearest integer)

(Given: $M g {(O H)}_{2}$ is assumed to dissociate completely in $H_{2} O$ ) [2025]