Answer to Question #94762 in Chemistry for Tshepiso

Question

a weak acid ha,having a ka of 1 x 10-9,ionises in water according to the equation ha=h + a-.what are the concentrations of all the species in a solution containing 0.1 mole of ha per cubic decimetre?

Answer 1

A weak acid dissociates according to the equation:

HA = H⁺ + A^-

According to the acid dissociation constant (K_a) concept:

K_a = [H⁺] × [A^-]/[HA],

where [H+], [A-], [HA] - molar concentrations of H+, A- and HA at equilibrium.

From here:

[HA] × K_a = [H⁺]×[A^-]

As [H⁺] = [A^-]:

[A-] = [H+] = ([HA] × K_a)^1/2

[HA] = n/V,

where n - number of mokes (0.1 mole), V - volume (1 dm³ = 1L).

As a result, HA concentration equals:

[HA] = 0.1 mole / 1L = 0.1 M

Concentrations of [A-] and [H+] equal:

[A^-] = [H⁺] = ([HA] × K_a)^1/2 = (0.1 M × 10^-9 )^1/2 = 10^-5 M

Answer: [HA] = 0.1 M, [H⁺] = 10^-5 M, [A^-] = 10^-5 M.