Question

Calculate the hydronium ion concentration in 0.120 M nitrous acid. ka( nitrous acid) =7.1x10^4. Use the ICE method to determone concentration of H3O+ = 9.23 x10^—3

Answer 1

Let a be the dissociation of the weak nitous acid(HA)
                            HA    H ⁺ + A^-

Initial conc.            c               0         0

Change                 -ca          +ca      +ca

Equb. conc.         c(1-a)          ca       ca

Dissociation constant , K_a   = ([H ⁺ ] [ A^-] ) / [HA]

                                         = ca x ca / ( c(1-a)

                                         = c a² / (1-a)

In the case of weak acids α is very small so 1-a is taken as 1

So K_a = ca²

==> a = √ ( K_a / c )

Given K_a = 7.1x10^-4

          c = concentration = 0.120 M

Plug the values we get a = 0.0769

So [H⁺] = ca = 0.120 x 0.0769

                    = 9.23x10^-3 M