I came across this question on one of the practice papers I have and isn't sure about the answer, any help would be appreciated. The following reaction occurs at constant temperature and constant volume in a closed system: CaCO3(s) + 2H(ion)(aq) + 2Cl(ion)(aq) -> CO2(g) + H2O(l) + Ca(ion)(aq) + 2Cl(ion)(aq) Changing which of the following would be useful in experimentally measuring the rate of this reaction? a) The mass of the system b) The pressure of the system c) The concentration of water d) the concentration of Cl ions I think that the answer should be b), since c) and d) is obviously incorrect. Water is the product, so increasing its concentration doesn't help to determine the rxn rate. Cl ions cancel out at either end, and I don't know what the heck mass of the entire system means in terms of the reaction. Does it mean increasing the mass of every substance in the equation? Plz help.