The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) is 1:1, so the moles of NaOH used in the reaction can be used to determine the moles of HCl in the solution. Using the volume and concentration of NaOH, you can calculate the moles of NaOH used. Then, using the balanced equation, you can determine the moles of HCl, which can be used to find the molarity of HCl in the solution.