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Electrochemistry question

2022 · Shift 2 · Q3
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Electrochemistry question

2022 · Shift 2 · Q3

JEE AdvancedChemistryElectrochemistryNumerical+3 / −1
Consider the strong electrolytes ZmXn,UmYpZ_{m} X_{n}, U_{m} Y_{p}Zm​Xn​,Um​Yp​ and VmXnV_{m} X_{n}Vm​Xn​. Limiting molar conductivity ( Λ0\Lambda^{0}Λ0) of UmYp\mathrm{U}_{\mathrm{m}} \mathrm{Y}_{\mathrm{p}}Um​Yp​ and VmXn\mathrm{V}_{\mathrm{m}} \mathrm{X}_{\mathrm{n}}Vm​Xn​ are 250 and 440 S cm2 mol−1440 \mathrm{~S} \mathrm{~cm}^{2} \mathrm{~mol}^{-1}440 S cm2 mol−1, respectively. The value of (m+n+p)(\mathrm{m}+\mathrm{n}+\mathrm{p})(m+n+p) is

Given:

Ion Zn+\mathrm{Z}^{\mathrm{n}+}Zn+ Up+\mathrm{U}^{\mathrm{p}+}Up+ Vn+\mathrm{V}^{\mathrm{n}+}Vn+ Xm−\mathrm{X}^{\mathrm{m}-}Xm− Ym−\mathrm{Y}^{\mathrm{m}-}Ym−
λ0( S cm2 mol−1)\lambda^{0}\left(\mathrm{~S} \mathrm{~cm}^{2} \mathrm{~mol}^{-1}\right)λ0( S cm2 mol−1) 50.050.050.0 25.025.025.0 100.0100.0100.0 80.080.080.0 100.0100.0100.0

λ0\lambda^{0}λ0 is the limiting molar conductivity of ions

The plot of molar conductivity (Λ\LambdaΛ) of ZmXnvs c1/2\mathrm{Z}_{\mathrm{m}} \mathrm{X}_{\mathrm{n}} v s\, \mathrm{c}^{1 / 2}Zm​Xn​vsc1/2 is given below.

JEE Advanced 2022 Paper 2 Online Chemistry - Electrochemistry Question 11 English
Numerical answer
View written solutionFree

Correct answer: 7

Step-by-Step Solution

  1. Understand Kohlrausch's Law

    Kohlrausch's law of independent migration of ions states that the limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte. For a general strong electrolyte AxByA_x B_yAx​By​, which dissociates into xxx cations Ay+A^{y+}Ay+ and yyy anions Bx−B^{x-}Bx−, the law is expressed as:

    Λ0(AxBy)=xλ0(Ay+)+yλ0(Bx−)\Lambda^0(A_x B_y) = x \lambda^0(A^{y+}) + y \lambda^0(B^{x-})Λ0(Ax​By​)=xλ0(Ay+)+yλ0(Bx−)

    where Λ0\Lambda^0Λ0 is the limiting molar conductivity of the electrolyte and λ0\lambda^0λ0 is the limiting molar conductivity of the individual ions.

  2. Formulate Equations for UmYpU_m Y_pUm​Yp​ and VmXnV_m X_nVm​Xn​

    We are given the limiting molar conductivities for two electrolytes and the ionic conductivities in a table. We can apply Kohlrausch's law to them.

    • For the electrolyte UmYpU_m Y_pUm​Yp​, the ions are mmm moles of Up+U^{p+}Up+ and ppp moles of Ym−Y^{m-}Ym−. Λ0(UmYp)=mλ0(Up+)+pλ0(Ym−)\Lambda^0(U_m Y_p) = m \lambda^0(U^{p+}) + p \lambda^0(Y^{m-})Λ0(Um​Yp​)=mλ0(Up+)+pλ0(Ym−) Substituting the given values: 250=m(25.0)+p(100.0)250 = m(25.0) + p(100.0)250=m(25.0)+p(100.0) Dividing the equation by 25, we get: 10=m+4p⋯(1)10 = m + 4p \quad \cdots(1)10=m+4p⋯(1)

    • For the electrolyte VmXnV_m X_nVm​Xn​, the ions are mmm moles of Vn+V^{n+}Vn+ and nnn moles of Xm−X^{m-}Xm−. Λ0(VmXn)=mλ0(Vn+)+nλ0(Xm−)\Lambda^0(V_m X_n) = m \lambda^0(V^{n+}) + n \lambda^0(X^{m-})Λ0(Vm​Xn​)=mλ0(Vn+)+nλ0(Xm−) Substituting the given values: 440=m(100.0)+n(80.0)440 = m(100.0) + n(80.0)440=m(100.0)+n(80.0) Dividing the equation by 20, we get: 22=5m+4n⋯(2)22 = 5m + 4n \quad \cdots(2)22=5m+4n⋯(2)

  3. Determine Λ0\Lambda^0Λ0 for ZmXnZ_m X_nZm​Xn​ from the Graph

    The plot of molar conductivity (/Lambda\\/Lambda/Lambda) versus the square root of concentration (c1/2c^{1/2}c1/2) for a strong electrolyte is described by the Debye-Hückel-Onsager equation: /Lambda=Λ0−Ac\\/Lambda = \Lambda^0 - A\sqrt{c}/Lambda=Λ0−Ac​, where Λ0\Lambda^0Λ0 is the limiting molar conductivity (the y-intercept).

    The graph shows a straight line with a negative slope. The equation for the line is provided on the graph as Λ=340−100c\Lambda = 340 - 100 \sqrt{c}Λ=340−100c​. By comparing this with the Debye-Hückel-Onsager equation, we can find the y-intercept, which occurs at c=0\sqrt{c}=0c​=0. Λ0=340 S cm2 mol−1\Lambda^0 = 340 \mathrm{~S} \mathrm{~cm}^{2} \mathrm{~mol}^{-1}Λ0=340 S cm2 mol−1 So, the limiting molar conductivity of ZmXnZ_m X_nZm​Xn​ is 340 S cm2 mol−1340 \mathrm{~S} \mathrm{~cm}^{2} \mathrm{~mol}^{-1}340 S cm2 mol−1.

  4. Formulate Equation for ZmXnZ_m X_nZm​Xn​

    • For the electrolyte ZmXnZ_m X_nZm​Xn​, the ions are mmm moles of Zn+Z^{n+}Zn+ and nnn moles of Xm−X^{m-}Xm−. Λ0(ZmXn)=mλ0(Zn+)+nλ0(Xm−)\Lambda^0(Z_m X_n) = m \lambda^0(Z^{n+}) + n \lambda^0(X^{m-})Λ0(Zm​Xn​)=mλ0(Zn+)+nλ0(Xm−) Substituting the values we have: 340=m(50.0)+n(80.0)340 = m(50.0) + n(80.0)340=m(50.0)+n(80.0) Dividing the equation by 10, we get: 34=5m+8n⋯(3)34 = 5m + 8n \quad \cdots(3)34=5m+8n⋯(3)
  5. Solve the System of Linear Equations

    We now have a system of three linear equations with three variables (m,n,pm, n, pm,n,p): (1) 10=m+4p10 = m + 4p10=m+4p (2) 22=5m+4n22 = 5m + 4n22=5m+4n (3) 34=5m+8n34 = 5m + 8n34=5m+8n

    Subtracting equation (2) from equation (3): (5m+8n)−(5m+4n)=34−22(5m + 8n) - (5m + 4n) = 34 - 22(5m+8n)−(5m+4n)=34−22 4n=124n = 124n=12 n=3n = 3n=3

    Substitute n=3n=3n=3 into equation (2): 22=5m+4(3)22 = 5m + 4(3)22=5m+4(3) 22=5m+1222 = 5m + 1222=5m+12 10=5m10 = 5m10=5m m=2m = 2m=2

    Substitute m=2m=2m=2 into equation (1): 10=2+4p10 = 2 + 4p10=2+4p 8=4p8 = 4p8=4p p=2p = 2p=2

    Thus, we have found the integer values: m=2,n=3,p=2m=2, n=3, p=2m=2,n=3,p=2.

  6. Calculate the Final Value

    The question asks for the value of (m+n+p)(m+n+p)(m+n+p). m+n+p=2+3+2=7m + n + p = 2 + 3 + 2 = 7m+n+p=2+3+2=7

    The final answer is 7.

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