Percentange ionisation of weak acid can be calculated using the formula:

Options

A. `100sqrt((K_(a))/(C ))`
B. `(100)/(1+10^((pK_(a)-pH))`
C. both (a) and (b)
D. none of these

Answer

C

Explanation

For weak acid dissociation equilibria, degree of dissociation `alpha` is given as:
`alpha = sqrt((K_(a))/(C ))`
`:. %alpha = 100sqrt((K_(a))/(C ))`
Also `K_(a)= ([H^(+)][A^(-)])/([HA])= ([H^(+)].Calpha)/(C(1-alpha))`
`= ([H^(+)].alpha)/((1-alpha))`
`log K_(a)= log H^(+)+log (alpha)/((1-alpha))`
or `pK_(a)=pH+log(1-alpha)/(alpha)`
or `pK_(a)-pH=log (1-alpha)/(alpha)`
`:. (1-alpha)/(alpha)=10^(pK_(a)-pH)`
or `(1)/(alpha)=10^(pK_(a)-pH)+1`
or `alpha=(1)/([1+10^(pK_(a)-pH))]`