A solution containing a monoprotic weak acid ( HA ) that is 5.60 % dissociated was found to have a pH of 1.78. What is the concentration of A − at equilibrium?

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!

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