James made the mistake of assuming that the density of water is 1 g/mL. However, the density of water is slightly less than 1 g/mL at room temperature. Therefore, adding 1 L of water to the 0.6000 g of KCl would result in a solution with a lower concentration than 0.6000 M.
To make a 0.6000 M solution of KCl, James should have calculated the amount of KCl needed to make the solution. The molar mass of KCl is 74.5513 g/mol, so 0.6000 moles of KCl would weigh 44.7308 g. Therefore, James should have added 44.7308 g of KCl to 1 L of water to make a 0.6000 M solution of KCl.