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JAMB Chemistry 1986 Question 25

Question 25

Water

The tabulated results below were obtained by titrating 10.0 cm3 of water with soap. The titration was repeated with the same sample of water after boiling.
Before boiling: Final (cm3) 25.0, Initial (cm3) 0.0
After boiling: Final (cm3) 20.0, Initial (cm3) 15.0
The ratio of permanent to temporary hardness is

  1. A.1:5
  2. B.1:4
  3. C.4:1
  4. D.5:1
Show the answer and explanation

Answer: B (1:4)

Explanation

Boiling removes only temporary hardness, so the 5.0 cm3 titre after boiling measures permanent hardness. Temporary hardness = 25.0 − 5.0 = 20.0, giving permanent : temporary = 5 : 20 = 1 : 4.