Solid Na 2 SO 4 is slowly added to a solution which is 0.020 M in Ba (NO 3 ) 2 and 0.020 M in Pb(NO 3 ) 2 . Assume that there is no increase in volume on adding Na 2 SO 4 . There preferential precipitation takes place. What is the concentration of first ion when the second ion starts to precipitate ? [K sp (BaSO 4 ) = 1.0 × 10 –10 and K sp (PbSO 4 ) = 1.6 × 10 –8 ]
Text Solution
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When BaSO 4 begins to precipitate
[SO 4 2– ] = 
= 
= 5.0 × 10 –9 M
When PbSO 4 begins to precipitate
[SO 4 2– ] = 
= 
= 8.0 × 10 –7 M
[SO 4 2– ] is less for BaSO 4 precipitation so BaSO 4 precipitates first when PbSO 4 begins to precipitate [SO 4 2– ] = 8.0 × 10 –7 M at that point concentration of Ba 2+ = 
= 
= 1.25 × 10 –4 M
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