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Solid N a 2 S O 4 is slowly added to a solution which is 0.020 M in B a N O 3 2 and 0.020 M in P b N O 3 2 . Assume that there is no increase in volume on adding N a 2 S O 4 . There preferential precipitation takes place. What is the concentration of Ba 2+ when PbSO 4 starts to precipitate? K s p B a S O 4 = 1.0 × 10 - 10 and K s p P b S O 4 = 1.6 × 10 - 8

Options

  1. A5.0 × 10 -9 M
  2. B8.0 × 10 -7 M
  3. C1.25 × 10 -4 M
  4. D1.95 × 10 -8 M

Correct answer

C. 1.25 × 10 -4 M

Step-by-step solution

When B a S O 4 begins to precipitate S O 4 2 - = K s p B a S O 4 B a 2 + = 1.0 × 1 0 - 10 0.020 = 5.0 × 10 - 9 M When P b S O 4 begins to precipitate S O 4 2 - = K s p P b S O 4 P b 2 + = 1.6 × 1 0 - 8 0.020 = 8.0 × 10 - 7 M S O 4 2 - is less for B a S O 4 precipitation so B a S O 4 precipitates first when P b S O 4 begins to precipitate S O 4 2 - = 8.0 × 10 - 7 M at that point concentration of B a 2 + = K s p B a S O 4 S O 4 2 - = 1.0 × 1 0 - 10 8.0 × 1 0 - 7 = 1.25 × 10 - 4 M

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