Why is it called "bicarbonate"? I'm helping to teach some kids about the baking soda/vinegar reaction, and was hoping to point out how the chemical names are just a direct translation of the chemical formula.  

However I was surprised to find there is no "2" in the formula for "sodium bicarbonate", $\ce{NaHCO3}$. 

Can someone explain where the "bi" comes in?
From Wikipedia


  The prefix bi in bicarbonate comes from an outdated naming system and is based on the observation that there is twice as much carbonate ($\ce{CO3}$) per sodium in sodium bicarbonate ($\ce{NaHCO3}$) as there is in sodium carbonate ($\ce{Na2CO3}$).


The IUPAC name for it agrees with its chemical formula: sodium hydrogen carbonate.