The prefix "bi" in "bicarbonate" comes from an outdated naming system and is based on the observation that there is twice as much carbonate (CO3) per sodium in sodium bicarbonate (NaHCO3) as there is in sodium carbonate (Na2CO3). The modern chemical formulas of these compounds express their precise chemical compositions (which were unknown when the names "sodium carbonate" and "sodium bicarbonate" were coined) as sodium hydrogen carbonate (NaHCO3) and sodium carbonate (Na2CO3). These names are unambiguous since sodium always has the +1 oxidation state and carbonate the −2 oxidation state.