Final answer:
At the endpoint of titration between HCl and NaOH, the moles of each are equal. To find the concentration of HCl, calculate the moles of NaOH used at the endpoint using its molarity and volume, then divide by the volume of HCl to find its molarity.
Step-by-step explanation:
To find the concentration of an HCl solution using titration with a NaOH solution, you first need to understand that at the endpoint of the titration, the moles of HCl will equal the moles of NaOH, as they react in a one-to-one molar ratio, according to the balanced chemical equation HCl(aq) + NaOH(aq) → NaCl(aq) + H2O. By using the molarity and volume of NaOH, you can calculate the moles of NaOH, and consequently, the moles of HCl. For example, if the volume of the NaOH necessary to reach the endpoint is 31.416 mL and the NaOH has a concentration of 0.1234 M, the moles of NaOH used are equal to the concentration of NaOH multiplied by the volume (in liters), which is also the number of moles of HCl in the unknown solution.
Following the formula Moles = Molarity × Volume, we have moles of HCl = 0.1234 M × 0.031416 L. To find the molarity of the HCl solution, you divide the moles of HCl by the volume of the HCl solution (in liters). If your HCl solution was 200.0 mL (or 0.200 L), then the concentration — molarity — of the original HCl solution would be the calculated number of moles of HCl divided by the volume of HCl in liters.