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5 g of Na 2 SO 4 was dissolved in x g of H 2 O . The change in freezing point was found to be 3 .82 o C . If Na 2 SO 4 is 81 . 5 % ionised, the value of x ( K f for water = 1 .86 o C kg mol - 1 ) is approximately: (molar mass of S = 32 g mol - 1 and that of Na = 23 g mol - 1 )

Options

  1. A25   g
  2. B65   g
  3. C15   g
  4. D45   g

Correct answer

D. 45   g

Step-by-step solution

Na 2 SO 4 → 2 Na + + SO 4 2 - i = 1 + 3 - 1   0 .815 = 2 .63 3 .82 = 1 .86 × 2 .63 × 5 × 1000 142 × x ∴   x = 1 .86 × 2 .63 × 5000 142 × 3 .82 = 45   gm

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