To work out an unknown concentration of 0.15 mL HCl:
Use the 1:1 ratio formula because one mole of HCl reacts with one mole of NaOH – HCl + NaOH → NaCl + H2O.
Multiply the molarity of the strong base NaOH by the volume of the NaOH (MB × VB = 0.500 M × 20.70 mL).
Divide this answer (10.35 M × mL) by the volume of the acid HCl (0.15 mL) MA = (MB × VB)/VA = (0.500 M × 20.70 mL)/0.15 mL = 0.690 M. The concentration is expressed as a number of moles per liter of solute.
Read more