What is the solubility of a sparignly soluble salt, PbSO4 in 0.01 MNa2SO4 solution ? (Ksp for PbSO4=1.6×10−8)
A
1.6×10−6molL−1
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B
2.6×10−9molL−1
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C
3.5×10−7molL−1
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D
0.4×10−9molL−1
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Solution
The correct option is A1.6×10−6molL−1 PbSO4 is a sparingly soluble salt , whereas Na2SO4 is a highly soluble salt. Let the solubility of PbSO4 be s. Then
PbSO4⇌Pb2++SO2−4 t=teqc−sss
Sodium sulphate is a strong electrolyte and is completely ionised. It shall provide SO2−4 ion concentration= 0.01M.
[Pb2+]=s [SO2−4]=(s+0.01)M
Ksp=[Pb2+][SO2−4]=s×(s+0.01)
Since Na2SO4 is a highly soluble .So s+0.01≈0.01, because of the sparingly soluble salt (PbSO4) where Ksp<<10−3 Given Ksp=1.6×10−8 ⇒1.6×10−8=0.01×s or s=1.6×10−80.01=1.6×10−6molL−1