What of an acid and a base is determined by how completely they dissociate in water
?

The strength of an acid and a base is determined by how completely they dissociate in water.

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Strong acids (like stomach acid) break down or dissociate in water. Weak acids  maintains their protons in water.

Explanation:

If an acid is strong in nature then upon dissolving in water it will completely dissociate into ions.

For example, HCl is a strong acid and when we dissolve it in water then it will dissociate into hydrogen () and chlorine () ions.

           

And, if an acid is weak in nature then it will partially dissociate into ions.

For example, acetic acid is a weak acid and it will dissociate as follows when dissolved in water.

Similarly, if a base is strong in nature then it will completely dissociate into ions.

For example, NaOH is a strong base and when it is added to water it will dissociate as follows.

       

And, when a base is weak in nature then it will partially dissociate upon dissolution in water.

For example, ammonia is a weak base and it will dissociate as follows when added to water.

     

The mass has escaped into a gas which you can not see so the law stays true.

257 grams h2o i think