Answer:
The concentration of [A⁻] is 0,0166M
Explanation:
In the equilibrium of a weak acid:
HA ⇄ A⁻ + H⁺
The solution of a weak acid contains, initially, the concentration of HA is Y. In equilibrium, the concentrations of each compound are:
[HA] = Y-x
[A⁻] = x
[H⁺] = x
That means [H⁺] = [A⁻].
The pH is defined as -log₁₀ [H⁺]. As the pH was 1,78:
1,78 = -log₁₀ [H⁺]
0,0166M = [H⁺]
As [H⁺] = [A⁻], the concentration of [A⁻] is 0,0166M
I hope it helps!
