In a 0.15 M solution of sodium chloride NaCl , the concentration of chloride ions Cl- is equal to the concentration of sodium ions Na+ , since NaCl dissociates completely in water into Na+ and Cl- ions. Therefore, the concentration of chloride ions in a 0.15 M NaCl solution is also 0.15 M.Potentiometry measurement is a technique used to determine the concentration of ions in a solution by measuring the potential difference voltage between two electrodes. In this case, you would use a chloride-selective electrode and a reference electrode to measure the potential difference. However, since we already know the concentration of NaCl, there is no need to perform potentiometry measurement to determine the concentration of chloride ions. The concentration of chloride ions in the 0.15 M NaCl solution is 0.15 M.