Carbonic acid reacts with water to yield bicarbonate ions and hydronium ions: h2co3+h2o? hco3? +h3o+ identify the conjugate acid-base pairs. drag each item to the appropriate bin.

Conjugate base pairs are acid and bases having common features. These features are the equal gain or loss of protons of the pairs. Conjugate pairs should always be one base and one acid. One would not exist without the other. Conjugate acids are the substances that gains protons while conjugates bases are those that loses protons. The substances in the equilibrium reaction that is given is identified as follows: 
HCO3^-      +     H2O <>   CO3^2-          +             H3O^+ 
 acid                   base         conjugate base           conjugate acid 

HCO3^- ion is an intermediate molecule of CO2 and CO3^2-. When we add OH- to HCO3^-, we produce CO3^2-. And when we add H+ to HCO3, we produce CO2. 

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