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JEE Main 201219 MayOnline
Exam
JEE Main
Year
2012
Date
19 May
Mode
Online
Subject
Chemistry
Chapter
Ionic Equilibrium

JEE Main 2012 · 19 May · Online · Chemistry · Ionic Equilibrium

If K_ s p of CaF_2 at 25^ C is 1.7 × 10^ -10 , the combination...

If KspK_{s p} of CaF2\mathrm{CaF}_2 at 25∘C25^{\circ} \mathrm{C} is 1.7×10−101.7 \times 10^{-10}, the combination amongst the following which gives a precipitate of CaF2\mathrm{CaF}_2 is

Options

  1. A1×10−2MCa2+1 \times 10^{-2} \mathrm{M} \mathrm{Ca}^{2+} and 1×10−3MF−1 \times 10^{-3} \mathrm{M} \mathrm{F}^{-}
  2. B1×10−4MCa2+1 \times 10^{-4} \mathrm{M} \mathrm{Ca}^{2+} and 1×10−4MF−1 \times 10^{-4} \mathrm{M} \mathrm{F}^{-}
  3. C1×10−2MCa2+1 \times 10^{-2} \mathrm{M} \mathrm{Ca}^{2+} and 1×10−5MF−1 \times 10^{-5} \mathrm{M} \mathrm{F}^{-}
  4. D1×10−3MCa2+1 \times 10^{-3} \mathrm{M} \mathrm{Ca}^{2+} and 1×10−5MF−1 \times 10^{-5} \mathrm{M} \mathrm{F}^{-}

Correct answer

A. 1×10−2MCa2+1 \times 10^{-2} \mathrm{M} \mathrm{Ca}^{2+} and 1×10−3MF−1 \times 10^{-3} \mathrm{M} \mathrm{F}^{-}

Step-by-step solution

When ionic product i.e. the product of the concentration of ions in the solution exceeds the value of solubility product, formation of precitpiate occurs. CaF2⇌Ca2++2F−Ionic product =[Ca2+][F−]2 when, [Ca2+]=1×10−2M[F−]2=(1×10−3)2M=1×10−6M∴[Ca2+][F−]2=(1×10−2)(1×10−6)=1×10−8 In  this case,  Ionic product (1×10−8)> solubility product (1.7×10−10)\begin{aligned} & \mathrm{CaF}_2 \rightleftharpoons \mathrm{Ca}^{2+}+2 \mathrm{ F}^{-} \\ & \text{Ionic product }=\left[\mathrm{Ca}^{2+}\right]\left[\mathrm{F}^{-}\right]^2 \\ & \text{ when, }\left[\mathrm{Ca}^{2+}\right]=1 \times 10^{-2} \mathrm{M} \\ & {\left[\mathrm{F}^{-}\right]^2=\left(1 \times 10^{-3}\right)^2 \mathrm{M} } \\ & =1 \times 10^{-6} \mathrm{M} \\ \therefore & {\left[\mathrm{Ca}^{2+}\right]\left[\mathrm{F}^{-}\right]^2=\left(1 \times 10^{-2}\right)\left(1 \times 10^{-6}\right)=1 \times 10^{-8} } \\ & \text{\text{ In } this case, } \\ & \text{ Ionic product }\left(1 \times 10^{-8}\right)> \\ & \text{ solubility product }\left(1.7 \times 10^{-10}\right)\end{aligned}\therefore$ Hence (a) is correct option.
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