To estimate the total mass of chlorine that was available for each trial, we need to calculate the mass of chlorine used in each trial. We can do this by subtracting the mass of sodium chloride produced from the mass of sodium used. The difference will give us the mass of chlorine used in each trial. Then we can add up all the masses of chlorine used in each trial to get an estimate of the total mass of chlorine that was available for each trial.