Equilibrium NEET PYQ PDF Download with Answers

Q 1 :
Phosphoric acid ionizes in three steps with their ionization constant values $K_{a_{1}}$ , $K_{a_{2}}$ and $K_{a_{3}}$ respectively, while $K$ is the overall ionization constant.

Which of the following statements are true?

A. $\log K = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}$
B. $H_{3} {PO}_{4}$ is a stronger acid than $H_{2} {PO}_{4}^{-}$ and ${HPO}_{4}^{2 -}$
C. $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$
D. $K_{a_{1}} = \frac{K_{a_{3}} + K_{a_{2}}}{2}$

Choose the correct answer from the options given below: [2025]

B, C and D only
A, B and C only
A and B only
A and C only

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(2)

$K = K_{a_{1}} \times K_{a_{2}} \times K_{a_{3}}$

Hence, $\log K = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}$

It is difficult to remove a proton from negatively charged ion, hence, $H_{3} {PO}_{4}$ is stronger acid than $H_{2} {PO}_{4}^{-}$ and ${HPO}_{4}^{2 -}$ .

Higher order ionisation constants $(K_{a_{2}}, K_{a_{3}})$ are smaller than the lower order ionisation constant $(K_{a_{1}})$ . Hence, $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$ .

Q 2 :
For a weak acid HA, the percentage of dissociation is nearly 1% at equilibrium. If the concentration of acid is 0.1 mol $L^{- 1}$ , then the correct option for its $K_{a}$ at the same temperature is [2023]

$1 \times 10^{- 4}$
$1 \times 10^{- 6}$
$1 \times 10^{- 5}$
$1 \times 10^{- 3}$

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(3)

$K_{a} = C α^{2}$

$K_{a} = (0.1) \times {(0.01)}^{2} = 1 \times 10^{- 5}$

Q 3 :
The $p K_{b}$ of dimethylamine and $p K_{a}$ of acetic acid are 3.27 and 4.77 respectively at $T (K)$ . The correct option for the pH of dimethylammonium acetate solution is [2021]

6.25
8.50
5.50
7.75

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(4)

For a salt of weak acid and weak base

$pH = 7 + \frac{1}{2} (p K_{a} - p K_{b})$

Given, $p K_{a} = 4.77, p K_{b} = 3.27$

$pH = 7 + \frac{1}{2} (4.77 - 3.27) = 7.75$

Q 4 :
Find out the solubility of $Ni {(OH)}_{2}$ in 0.1 M NaOH. Given that the ionic product of $Ni {(OH)}_{2}$ is $2 \times 10^{- 15}$ . [2020]

$2 \times 10^{- 13}$ M
$2 \times 10^{- 8}$ M
$1 \times 10^{- 13}$ M
$1 \times 10^{8}$ M

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(1)

$\underset{s}{{Ni(OH)}_{2}} ⇌ \underset{s}{{Ni}^{2 +}} + \underset{2 s}{2 {OH}^{-}}$

where $s$ is the solubility of $Ni {(OH)}_{2}$ .

$\underset{0.1 M}{NaOH} ⇌ \underset{0.1 M}{{Na}^{+}} + \underset{0.1 M}{{OH}^{-}}$

$[{OH}^{-}] = 2 s + 0.1 \approx 0.1 (∵ 2 s < < < 0.1)$

Ionic product of $Ni {(OH)}_{2}$ $= [{Ni}^{2 +}] {[{OH}^{-}]}^{2} = 2 \times 10^{- 15} = s {(0.1)}^{2}$

$s = \frac{2 \times 10^{- 15}}{0.1 \times 0.1} = 2 \times 10^{- 13}$ $M$

Q 5 :
The pH of 0.01 M $N a O H_{(a q)}$ solution will be [2019]

7.01
2
12
9

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(3)

$\underset{0.01 M}{NaOH} \to {Na}^{+} + \underset{0.01 M}{{OH}^{-}}$

$∴$ $[{OH}^{-}] = 0.01$ $M$

$∴$ $pOH = - \log [{OH}^{-}] = - \log (0.01) = 2$

$∴$ $pH = 14 - pOH = 14 - 2 = 12$

Q 6 :
Following solutions were prepared by mixing different volumes of NaOH and HCl of different concentrations:

A. 60 mL $\frac{M}{10} HCl$ + 40 mL $\frac{M}{10}$ NaOH

B. 55 mL $\frac{M}{10} HCl$ + 45 mL $\frac{M}{10}$ NaOH

C. 75 mL $\frac{M}{5} HCl$ + 25 mL $\frac{M}{5}$ NaOH

D. 100 mL $\frac{M}{10} HCl$ + 100 mL $\frac{M}{10}$ NaOH

pH of which one of them will be equal to 1? [2018]

B
A
D
C

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(4)

pH = 1, so $[H^{+}] = 10^{- 1}$

For acid-base mixture: $N_{1} V_{1} - N_{2} V_{2} = N_{3} V_{3}$

(For NaOH and HCl, Normality = Molarity)

A. $M_{1} (H^{+}) = \frac{60 \times \frac{1}{10} - 40 \times \frac{1}{10}}{100} = 2 \times 10^{- 2} M i.e. pH = 1.698 \approx 1.7$

B. $M_{2} (H^{+}) = \frac{55 \times \frac{1}{10} - 45 \times \frac{1}{10}}{100} = \frac{1}{100} = 10^{- 2} M i.e. pH = 2$

C. $M_{3} (H^{+}) = \frac{75 \times \frac{1}{5} - 25 \times \frac{1}{5}}{100} = 10^{- 1} M i.e. pH = 1$

D. $M_{4} (H^{+}) = \frac{100 \times \frac{1}{10} - 100 \times \frac{1}{10}}{200} = 0 i.e. pH = 7$

Q 7 :
The percentage of pyridine $(C_{5} H_{5} N)$ that forms pyridinium ion $(C_{5} H_{5} N H)$ in a 0.10 M aqueous pyridine solution ( $K_{b}$ for $C_{5} H_{5} N = 1.7 \times 10^{- 9}$ ) is [2016]

0.0060%
0.013%
0.77%
1.6%

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(2)

$\underset{0.10 M}{C_{5} H_{5} N} + H_{2} O ⇌ C_{5} H_{5} \overset{+}{N} H + {OH}^{-}$

$α = \sqrt{\frac{K_{b}}{C}} = \sqrt{\frac{1.7 \times 10^{- 9}}{0.10}} = 1.30 \times 10^{- 4}$

$∴$ $Percentage of pyridine that forms pyridinium ion = 1.30 \times 10^{- 4} \times 100 = 0.013 %$

Q 8 :
What is the pH of the resulting solution when equal volumes of 0.1 M NaOH and 0.01 M HCl are mixed? [2015]

2.0
7.0
1.04
12.65

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(4)

One mole of NaOH is completely neutralised by one mole of HCl.

Hence, 0.01 mole of NaOH will be completely neutralised by 0.01 mole of HCl.

$\Rightarrow$ NaOH left unneutralised = 0.1 - 0.01 = 0.09 mol

As equal volumes of two solutions are mixed,

${[OH]}^{-} = \frac{0.09}{2} = 0.045$ $M$

$\Rightarrow pOH = - \log (0.045) = 1.35 ∴ pH = 14 - 1.35 = 12.65$

Q 9 :
Which of the following salts will give highest pH in water? [2014]

$KCl$
$NaCl$
${Na}_{2} {CO}_{3}$
${CuSO}_{4}$

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(3)

${Na}_{2} {CO}_{3}$ , which is a salt of NaOH (strong base) and $H_{2} {CO}_{3}$ (weak acid), will produce a basic solution with pH greater than 7.

Topic Question Set

Q 2 :
For a weak acid HA, the percentage of dissociation is nearly 1% at equilibrium. If the concentration of acid is 0.1 mol $L^{- 1}$ , then the correct option for its $K_{a}$ at the same temperature is [2023]

Q 3 :
The $p K_{b}$ of dimethylamine and $p K_{a}$ of acetic acid are 3.27 and 4.77 respectively at $T (K)$ . The correct option for the pH of dimethylammonium acetate solution is [2021]

Q 4 :
Find out the solubility of $Ni {(OH)}_{2}$ in 0.1 M NaOH. Given that the ionic product of $Ni {(OH)}_{2}$ is $2 \times 10^{- 15}$ . [2020]

Q 5 :
The pH of 0.01 M $N a O H_{(a q)}$ solution will be [2019]

Q 7 :
The percentage of pyridine $(C_{5} H_{5} N)$ that forms pyridinium ion $(C_{5} H_{5} N H)$ in a 0.10 M aqueous pyridine solution ( $K_{b}$ for $C_{5} H_{5} N = 1.7 \times 10^{- 9}$ ) is [2016]

Q 8 :
What is the pH of the resulting solution when equal volumes of 0.1 M NaOH and 0.01 M HCl are mixed? [2015]

Q 9 :
Which of the following salts will give highest pH in water? [2014]

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Topic Question Set

Q 2 : For a weak acid HA, the percentage of dissociation is nearly 1% at equilibrium. If the concentration of acid is 0.1 mol L-1, then the correct option for its Ka at the same temperature is [2023]

Q 3 : The pKb of dimethylamine and pKa of acetic acid are 3.27 and 4.77 respectively at T(K). The correct option for the pH of dimethylammonium acetate solution is [2021]

Q 4 : Find out the solubility of Ni(OH)2 in 0.1 M NaOH. Given that the ionic product of Ni(OH)2 is 2×10-15. [2020]

Q 5 : The pH of 0.01 M NaOH(aq) solution will be [2019]

Q 6 : Following solutions were prepared by mixing different volumes of NaOH and HCl of different concentrations: A. 60 mL M10HCl + 40 mL M10 NaOH B. 55 mL M10HCl + 45 mL M10 NaOH C. 75 mL M5HCl + 25 mL M5 NaOH D. 100 mL M10HCl + 100 mL M10 NaOH pH of which one of them will be equal to 1? [2018]

Q 7 : The percentage of pyridine (C5H5N) that forms pyridinium ion (C5H5NH) in a 0.10 M aqueous pyridine solution (Kb for C5H5N=1.7×10-9) is [2016]

Q 8 : What is the pH of the resulting solution when equal volumes of 0.1 M NaOH and 0.01 M HCl are mixed? [2015]

Q 9 : Which of the following salts will give highest pH in water? [2014]

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Q 2 :
For a weak acid HA, the percentage of dissociation is nearly 1% at equilibrium. If the concentration of acid is 0.1 mol $L^{- 1}$ , then the correct option for its $K_{a}$ at the same temperature is [2023]

Q 3 :
The $p K_{b}$ of dimethylamine and $p K_{a}$ of acetic acid are 3.27 and 4.77 respectively at $T (K)$ . The correct option for the pH of dimethylammonium acetate solution is [2021]

Q 4 :
Find out the solubility of $Ni {(OH)}_{2}$ in 0.1 M NaOH. Given that the ionic product of $Ni {(OH)}_{2}$ is $2 \times 10^{- 15}$ . [2020]

Q 5 :
The pH of 0.01 M $N a O H_{(a q)}$ solution will be [2019]

Q 7 :
The percentage of pyridine $(C_{5} H_{5} N)$ that forms pyridinium ion $(C_{5} H_{5} N H)$ in a 0.10 M aqueous pyridine solution ( $K_{b}$ for $C_{5} H_{5} N = 1.7 \times 10^{- 9}$ ) is [2016]

Q 8 :
What is the pH of the resulting solution when equal volumes of 0.1 M NaOH and 0.01 M HCl are mixed? [2015]

Q 9 :
Which of the following salts will give highest pH in water? [2014]