In this case, the weak ionization reaction for formic acid is:
In such a way, we simply recall the law of mass action in order to represent the weak ionization constant, Ka, for such process, by taking into account that the concentration of products is divided over the concentration of reactants as shown below:
HA is a weak acid so there should be little amount of HA and then H+ and A- in the beaker so the last one is true a weak acid never converts completely to H+ and A-