The following is from the portion of John Palmer's website talking about water and water reports which can be found here
Molecular Weight = 61.0
Equivalent Weight = 61.0
Brewing Range = 0-50 ppm for pale, base-malt only beers.
50-150 ppm for amber colored, toasted malt beers, 150-250 ppm for dark, roasted malt beers.
The carbonate family of ions are the big players in determining brewing water chemistry. Carbonate (CO3-2), is an alkaline ion, raising the pH, and neutralizing dark malt acidity. Its cousin, bicarbonate (HCO3-1), has half the buffering capability but actually dominates the chemistry of most brewing water supplies because it is the principal form for carbonates in water with a pH less than 8.4. Carbonate itself typically exists as less than 1% of the total carbonate/bicarbonate/carbonic acid species until the pH exceeds 8.4. There are two methods the homebrewer can use to bring the bicarbonate level down to the nominal 50 - 150 ppm range for most pale ales, or even lower for light lagers such as Pilsener. These methods are boiling, and dilution.
Carbonate can be precipitated (ppt) out as Calcium Carbonate (CaCO3) by aeration and boiling according to the following reaction:
2HCO3-1 + Ca+2 + O2 gas --> CaCO3 (ppt) + H2O + CO2 gas
where oxygen from aeration acts as a catalyst and the heat of boiling prevents the carbon dioxide from dissolving back into the water to create carbonic acid.
Dilution is the easiest method of producing low carbonate water. Use distilled water from the grocery store (often referred to as Purified Water for use in steam irons) in a 1:1 ratio, and you will effectively cut your bicarbonate levels in half, although there will be a minor difference due to buffering reactions. Bottom Line: if you want to make soft water from hard water (e.g. to brew a Pilsener), dilution with distilled water is the best route.