Chemical Thermodynamics Question 414

Question: The enthalpy of neutralisation of a weak acid in 1 M solution with a strong base is $ -56.1kcalmo{{l}^{-1}}. $ If the enthalpy of ionisation of acid is $ 1.5kcalmo{{l}^{-1}} $ and enthalpy of neutralisation of the strong acid with a strong base is $ -57.3kJe{{q}^{-1}} $ . What is the % ionisation of the weak acid in molar solution (assume the acid is monobasic)

Options:

A) 25

B) 20

C) 15

D) 10

Show Answer

Answer:

Correct Answer: B

Solution:

  • The enthalpy of ionisation of weak acid is given by $ \Delta H_{ion}(HA) $

$ =\Delta H_{N} $ (weak acid / strong base) - $ \Delta H_{N} $ (strong acid / strong base) $ =-56.1-( -57.3 )=1.2kJ,mo{{l}^{-1}} $

$ \Delta {H_{(ionisation)}}=1.5,kJmo{{l}^{\text{-}1}} $

Hence % ionisation in 1 M solution $ =\frac{(1.5-1.2)}{1.5}\times 100=20% $ %



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