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25/10/2021 12:13 pm
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A solution is saturated with respect to SrCO3 and SrF2. The [CO32-] was found to be 1.2 × 10-3 M. The concentration of F- in the solution would be
Given Ksp of SrCO3 = 7.0 x 10-10 M2,
Ksp of SrF2 = 7.9 x 10-10 M3,
(a) 1.3 × 10-3 M
(b) 2.6 × 10-2 M
(c) 3.7 × 10-2 M
(d) 5.8 × 10-7 M
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25/10/2021 12:26 pm
Correct answer: (c) 3.7 × 10-2 M
Explanation:
The two Ksp values do not differ very much. So it is a case of simultaneous equilibria, where the concentration of any species can not be neglected.
\(\frac{[Sr^{2+}][F^-]^2}{[Sr^{2+}][CO_3^{2-}]}\)
= \(\frac{K_{{sp}_{SrF_2}}}{K_{{sp}_{SrCO_2}}}\)
= \(\frac{7.9 \times 10^{-10}}{7.0 \times 10^{-10}}\) = 1.128
∴ [F-]2 = 1.128 x 1.2 x 10-3 = 13.5 x 10-4
∴ [F-]2 = (13.5 x 10-4)1/2
= 3.674 x 10-2 = 3.7 x 10-2 M