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Ionic Equilibrium question

2022 · 29 Jul · Shift 1 · Q20
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  5. /2022 · 29 Jul · Shift 1 · Q20

Ionic Equilibrium question

2022 · 29 Jul · Shift 1 · Q20

JEE MainChemistryIonic EquilibriumNumerical+4 / −1
If the solubility product of PbS is 8 ×\times× 10 −-− 28, then the solubility of PbS in pure water at 298 K is x ×\times× 10 −-− 16 mol L −-− 1. The value of x is ‾\underline{\hspace{2cm}}​. (Nearest Integer) [Given : 2\sqrt22​ = 1.41]
Numerical answer
View written solutionFree

Correct answer: 282

  1. Write the dissolution equilibrium

For lead sulfide: PbS(s)⇌Pb2+(aq)+S2−(aq)\text{PbS}(s) \rightleftharpoons \text{Pb}^{2+}(aq) + \text{S}^{2-}(aq)PbS(s)⇌Pb2+(aq)+S2−(aq)

If its solubility in pure water is sss mol L−1^{-1}−1, then at equilibrium: [Pb2+]=s,[S2−]=s[\text{Pb}^{2+}] = s, \qquad [\text{S}^{2-}] = s[Pb2+]=s,[S2−]=s

  1. Use the solubility product expression

Given: Ksp=[Pb2+][S2−]=s⋅s=s2K_{sp} = [\text{Pb}^{2+}][\text{S}^{2-}] = s \cdot s = s^2Ksp​=[Pb2+][S2−]=s⋅s=s2

So, s2=8×10−28s^2 = 8 \times 10^{-28}s2=8×10−28

s=8×10−28s = \sqrt{8 \times 10^{-28}}s=8×10−28​

  1. Simplify the square root

s=8×10−14s = \sqrt{8} \times 10^{-14}s=8​×10−14

Now, 8=22\sqrt{8} = 2\sqrt{2}8​=22​

Given 2=1.41\sqrt{2} = 1.412​=1.41, 8=2×1.41=2.82\sqrt{8} = 2 \times 1.41 = 2.828​=2×1.41=2.82

Hence, s=2.82×10−14 mol L−1s = 2.82 \times 10^{-14}\ \text{mol L}^{-1}s=2.82×10−14 mol L−1

  1. Match with the required form

We need: s=x×10−16 mol L−1s = x \times 10^{-16}\ \text{mol L}^{-1}s=x×10−16 mol L−1

Since 2.82×10−14=282×10−162.82 \times 10^{-14} = 282 \times 10^{-16}2.82×10−14=282×10−16

therefore, x=282x = 282x=282

  1. Nearest integer

282\boxed{282}282​

  1. Comparison with stored answer

Stored correct answer = 282282282.

Our derived answer also is 282282282, so they agree.

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