Amonoprotic weak acid, ha , dissociates in water according to the reaction ha ( aq ) − ⇀ ↽ − h + ( aq ) + a − ( aq ) the equilibrium concentrations of the reactants and products are [ ha ] = 0.170 m , [ h + ] = 2.00 × 10 − 4 m , and [ a − ] = 2.00 × 10 − 4 m . calculate the value of p k a for the acid ha .