The enthalpy of neutralisation of a weak acid in $$1\,M$$ solution with a strong base is $$ - 56.1\,kcal\,mo{l^{ - 1}}.$$ If the enthalpy of ionisation of acid is $$1.5\,kcal\,mo{l^{ - 1}}$$ and enthalpy of neutralisation of the strong acid with a strong base is $$ - 57.3\,kJ\,e{q^{ - 1}}.$$ What is the $$\% $$ ionisation of the weak acid in molar solution ( assume the acid is monobasic )
A.
25
B.
20
C.
15
D.
10
Answer :
20
Solution :
The enthalpy of ionisation of weak acid is given by
$$\Delta {H_{ion}}\left( {HA} \right)$$
$$ = \Delta {H_N}$$ ( weak acid / strong base ) $$ -\, \,\Delta {H_N}$$ ( strong acid / strong base )
$$\eqalign{
& = - 56.1 - \left( { - 57.3} \right) = 1.2\,kJ\,mo{l^{ - 1}} \cr
& \Delta {H_{{\text{(ionisation)}}}} = 1.5\,kJ\,mo{l^{ - 1}} \cr} $$
Hence % ionisation in $$1 M$$ solution
$$\eqalign{
& = \frac{{\left( {1.5 - 1.2} \right)}}{{1.5}} \times 100 \cr
& = 20\% \cr} $$
Releted MCQ Question on Physical Chemistry >> Chemical Thermodynamics
Releted Question 1
The difference between heats of reaction at constant pressure and constant volume for the reaction : $$2{C_6}{H_6}\left( l \right) + 15{O_{2\left( g \right)}} \to $$ $$12C{O_2}\left( g \right) + 6{H_2}O\left( l \right)$$ at $${25^ \circ }C$$ in $$kJ$$ is
$${\text{The}}\,\Delta H_f^0\,{\text{for}}\,C{O_2}\left( g \right),\,CO\left( g \right)\,$$ and $${H_2}O\left( g \right)$$ are $$-393.5,$$ $$-110.5$$ and $$ - 241.8\,kJ\,mo{l^{ - 1}}$$ respectively. The standard enthalpy change ( in $$kJ$$ ) for the reaction $$C{O_2}\left( g \right) + {H_2}\left( g \right) \to CO\left( g \right) + {H_2}O\left( g \right)\,{\text{is}}$$