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Chemical Equilibrium question

2021 · 26 Aug · Shift 2 · Q14
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Chemical Equilibrium question

2021 · 26 Aug · Shift 2 · Q14

JEE MainChemistryChemical EquilibriumNumerical+4 / −1
The equilibrium constant Kc at 298 K for the reaction A + B ⇌\rightleftharpoons⇌ C + D is 100. Starting with an equimolar solution with concentrations of A, B, C and D all equal to 1M, the equilibrium concentration of D is ‾\underline{\hspace{2cm}}​×\times× 10 −-− 2 M. (Nearest integer)
Numerical answer
View written solutionFree

Correct answer: 182

  1. Given reaction and equilibrium constant

For the reaction A+B⇌C+DA + B \rightleftharpoons C + DA+B⇌C+D we have Kc=100K_c = 100Kc​=100

At the start: [A]0=[B]0=[C]0=[D]0=1 M[A]_0 = [B]_0 = [C]_0 = [D]_0 = 1\,\text{M}[A]0​=[B]0​=[C]0​=[D]0​=1M

We need the equilibrium concentration of DDD.


  1. Check the reaction quotient initially

The initial reaction quotient is Qc=[C][D][A][B]=(1)(1)(1)(1)=1Q_c = \frac{[C][D]}{[A][B]} = \frac{(1)(1)}{(1)(1)} = 1Qc​=[A][B][C][D]​=(1)(1)(1)(1)​=1

Since Qc<KcQ_c < K_cQc​<Kc​ that is, 1<1001 < 1001<100 so the reaction will proceed in the forward direction.


  1. Let the forward change be xxx

Then at equilibrium:

[A]=1−x[A] = 1 - x[A]=1−x [B]=1−x[B] = 1 - x[B]=1−x [C]=1+x[C] = 1 + x[C]=1+x [D]=1+x[D] = 1 + x[D]=1+x

Using the equilibrium constant expression: Kc=[C][D][A][B]=(1+x)(1+x)(1−x)(1−x)=100K_c = \frac{[C][D]}{[A][B]} = \frac{(1+x)(1+x)}{(1-x)(1-x)} = 100Kc​=[A][B][C][D]​=(1−x)(1−x)(1+x)(1+x)​=100

So, (1+x1−x)2=100\left(\frac{1+x}{1-x}\right)^2 = 100(1−x1+x​)2=100

Taking positive square root: 1+x1−x=10\frac{1+x}{1-x} = 101−x1+x​=10


  1. Solve for xxx

1+x=10(1−x)1 + x = 10(1 - x)1+x=10(1−x) 1+x=10−10x1 + x = 10 - 10x1+x=10−10x 11x=911x = 911x=9 x=911x = \frac{9}{11}x=119​

Thus, [D]eq=1+x=1+911=2011[D]_{eq} = 1 + x = 1 + \frac{9}{11} = \frac{20}{11}[D]eq​=1+x=1+119​=1120​

So, [D]eq=1.81818… M[D]_{eq} = 1.81818\ldots\,\text{M}[D]eq​=1.81818…M


  1. Express in the required form

We need [D]eq=‾×10−2 M[D]_{eq} = \underline{\hspace{2cm}} \times 10^{-2}\,\text{M}[D]eq​=​×10−2M

Now, 1.81818 M=181.818×10−2 M1.81818\,\text{M} = 181.818\times 10^{-2}\,\text{M}1.81818M=181.818×10−2M

Nearest integer =182= 182=182.


  1. Comparison with stored answer

Our derived answer is 182, which matches the stored correct answer.

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