1. The problem statement, all variables and given/known data A sample of hard water contains 305 ppm of HCO3- ions. What is the minimum mass of CaO required to remove HCO3- completely from 1 kg of water sample? 2. Relevant equations Ca 2+ + 2HCO3 + CaO = 2CaCO3 + H2O 3. The attempt at a solution Since the HCO3- ions are 305 parts per million, it can be assumed that there are 0.305 g of the ions in 1 kg (or 1 L as the density is 1) of water. (I don't know if this assumption is correct) If I use the equation as above, then I get 0.1525 g of CaO which is not correct. I think that the whole trouble is with the ppm part. Can someone give me a clue?