The pH of 0.1 M monobasic acid is 4.50 . Calculate the concentration of species H^(+), A^(-)and HA at equilibrium . Also , Determine the value of K_a and pK_a of themonobasic acid.

The pH of 0.1 M monobasic acid is 4.50 . Calculate the concentration o

1.	The pH of 0.1 M monobasic acid is 4.50 . Calculate the concentration of species H^(+), A^(-)and HA at equilibrium . Also , Determine the value of K_a and pK_a of themonobasic acid.
Answer» Solution :pH=4.5 =-log `[H^+]` `therefore log [H^+]=-4.5 = bar5.5` `therefore [H^+]` =Antilog `bar5.5=3.1623xx10^(-5)` M `approx 3.16xx10^(-5)` M The ionic EQUILIBRIUM of weak acid HA is under : `{:(,HA_((aq)) + ,H_2O_((l)) hArr , H_3O_((aq))^(+)+, A_((aq))^(-)),("Initial", 0.1M, -,0,0),("Equilibrium" , (0.1-alpha)M, ,alphaM, alphaM):}` `approx` 0.1 M So, `[H^+]=[A^-]=3.16xx10^(-5)` M At equilibrium [HA]=`(0.1 - alpha)` =`(0.1-3.16xx10^(-5))` `approx` 0.1 M Value of `alpha` is very small so, neglect. `K_a=([H^+][A^-])/([HA])=((3.16xx10^(-5))(3.16xx10^(-5)))/0.1` `=9.9856xx10^(-9)` `approx 10xx10^(-9) approx 1.0xx10^(-8)` `pK_a =-log K_a=-log (1.0xx10^(-8))` =-(0.0-8) = + 8