Problem: A 0.142 M solution of a monoprotic acid has a percent dissociation of 1.60%. Determine the acid ionization constant (Ka) for the acid.

We’re being asked to calculate the acid ionization constant (K_a) for a monoprotic acid if a 0.142 M solution has a percent dissociation of 1.60%.

Recall that the percent ionization is given by: 

(Let HA → monoprotic acid)

$\boxed{\mathbf{\%}\mathbf{ }\mathbf{ionization}\mathbf{=}\frac{\mathbf{\left[}{\mathbf{H}}_{\mathbf{3}}{\mathbf{O}}^{\mathbf{+}}\mathbf{\right]}}{\mathbf{[}\mathbf{HA}{\mathbf{]}}_{\mathbf{initial}}}\mathbf{\times }\mathbf{100}}$

Remember that weak acids partially dissociate in water and that acids donate H⁺ to the base (water in this case). 

The dissociation of HA is as follows:

HA(aq) + H₂O(l) ⇌ H₃O⁺(aq) + A^–(aq)