Dissociation of a gas A_2 takes place… - JEE Main 2026 Chemistry PYQ

JEE Main 2026 — Chemistry Physical Chemistry

2026integerhard

Dissociation of a gas $\mathrm{A}_{2}$ takes place according to the following chemical reaction. At equilibrium, the total pressure is 1 bar at 300 K.

$\mathrm{A}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{~A}(\mathrm{~g})$

The standard Gibbs energy of formation of the involved substances has been provided below:

$\begin{array}{|c|c|} \hline \text { Substance } & \Delta \mathrm{G}_{\mathrm{f}}^{\circ} / \mathrm{kJ} \mathrm{~mol}^{-1} \\ \hline \mathrm{~A}_2 & -100.00 \\ \hline \mathrm{~A} & -50.832 \\ \hline \end{array}$

The degree of dissociation of $\mathrm{A}_{2}(\mathrm{~g})$ is given by $\left(x \times 10^{-2}\right)^{1 / 2}$ where $x=$ $\_\_\_\_$ . (Nearest integer).

[Given: $\mathrm{R}=8 \mathrm{~J} \mathrm{~mol}^{-1} \mathrm{~K}^{-1}, \log 2=0.3010, \log 3=0.48$ ]

Assume degree of dissociation is not negligible.

Official previous-year question

Held on 22 Jan 2026 · Verified 6 Jul 2026.

Did you get this right?

Solution

For the dissociation reaction $A_2(g) \rightleftharpoons 2A(g)$ , first calculate the standard Gibbs free energy:

$\Delta G°_{rxn} = 2(-50.832) - (-100.00) = -1.664$ kJ/mol.

Using $\Delta G° = -RT \ln K_p$ :

$-1664 = -8 \times 300 \times \ln K_p$ , giving $K_p = e^{0.6933} = 2$ .

For dissociation with degree of dissociation $\alpha$ , starting with 1 mole of $A_2$ at 1 bar total pressure:

At equilibrium, moles are $(1-\alpha)$ for $A_2$ and $2\alpha$ for $A$ , with total $(1+\alpha)$ moles.

$K_p = \frac{4\alpha^2}{1-\alpha^2} = 2$ .

Solving: $2(1-\alpha^2) = 4\alpha^2$ gives $\alpha^2 = 1/3$ or $\alpha = 0.577$ .

Since degree of dissociation is $(x \times 10^{-2})^{1/2}$ :

$(x \times 10^{-2})^{1/2} = 0.577$ gives $x \times 10^{-2} = 0.333$ , so $x = 33.3 \approx 33$ .

Your note

JEE Main Physical Chemistry in other years

Physical Chemistry 2025 Physical Chemistry 2024 Physical Chemistry 2023 Physical Chemistry 2022 Physical Chemistry 2021 Physical Chemistry 2020 Physical Chemistry 2019 Physical Chemistry 2018 Physical Chemistry 2017 Physical Chemistry 2016 Physical Chemistry 2015 Physical Chemistry 2014

More JEE Main Physical Chemistry questions

What is the ratio of wave number of first line (lowest energy line) of Balmer series of H atomic spectrum to first line of its Brackett series?

2026medium

![JEE Main 2026 Chemistry, Structure of Atom — question figure 1](https://prepforbharat.s3.ap-south-1.amazonaws.com/exam/615f0e999476412f48314daf/Chemistry/images/Structure_of_Atom/6983c1ae8bfdbb24356397c7/question_1__q_6983c1ae8bfdbb24356397c7__cdn-question-pool.getmarks.app__28S1_q_64_1__80fd8624a7_final_ppt_sync.png) ![JEE Main 2026 Chemistry, Structure of Atom — question figure 2](https://prepforbharat.s3.ap-south-1.amazonaws.com/exam/615f0e999476412f48314daf/Chemistry/images/Structure_of_Atom/6983c1ae8bfdbb24356397c7/question_2__q_6983c1ae8bfdbb24356397c7__cdn-question-pool.getmarks.app__28S1_q_64_2__04a1587871_final_ppt_sync.png) Which of the following point in Figure 2 most accurately represents the nodal surface as shown in Figure 1?

2026medium

At 298 K, the mole percentage of $\mathrm{N}_{2}(\mathrm{~g})$ in air is $80 \%$. Water is in equilibrium with air at a pressure of 10 atm. What is the mole fraction of $\mathrm{N}_{2}(\mathrm{~g})$ in water at 298 K ? ($\mathrm{K}_{\mathrm{H}}$ for $\mathrm{N}_{2}$ is $6.5 \times 10^{7} \mathrm{~mm} \mathrm{Hg}$)

2026medium

Electricity is passed through an acidic solution of $\mathrm{Cu}^{2+}$ till all the $\mathrm{Cu}^{2+}$ was exhausted, leading to the deposition of 300 mg of Cu metal. However, a current of 600 mA was continued to pass through the same solution for another 28 minutes by keeping the total volume of the solution fixed at 200 mL. The total volume of oxygen evolved at STP during the entire process is $\_\_\_\_$ mL. (Nearest integer) [Given: $\mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Cu}(\mathrm{s}) \mathrm{E}_{\text {red }}^{\mathrm{o}}=+0.34 \mathrm{~V}$ $\mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \rightarrow 2 \mathrm{H}_{2} \mathrm{O} \mathrm{E}_{\text {red }}^{\mathrm{o}}=+1.23 \mathrm{~V}$ Molar mass of $\mathrm{Cu}=63.54 \mathrm{~g} \mathrm{~mol}^{-1}$ Molar mass of $\mathrm{O}_{2}=32 \mathrm{~g} \mathrm{~mol}^{-1}$ Faraday Constant $=96500 \mathrm{C} \mathrm{mol}^{-1}$ Molar volume at $\mathrm{STP}=22.4 \mathrm{~L}$]

2026hard

Arrange the following resultant mixtures in increasing order of their pH values A. $10$ mL $0.2$ M Ca(OH)$_2$ + $25$ mL $0.1$ M HCl B. $10$ mL $0.01$ M H$_2$SO$_4$ + $10$ mL $0.01$ M Ca(OH)$_2$ C. $10$ mL $0.1$ M H$_2$SO$_4$ + $10$ mL $0.1$ M KOH Choose the correct answer from the options given below:

2026medium

View all Physical Chemistry questions

Keep practicing Physical Chemistry

Work through every JEE Main Physical Chemistry PYQ, year by year.

Browse all Chemistry topics

Did you get this right?

Solution

Your note

JEE Main Physical Chemistry in other years

More JEE Main Physical Chemistry questions

Other Chemistry topics for JEE Main

Organic Chemistry

Inorganic Chemistry

Keep practicing Physical Chemistry