The balanced chemical reaction is:
HCl + NaOH -> NaCl + H2O
We see that there is a 1:1 mole correspondence
First calculate the moles of NaOH:
moles NaOH = 0.100 M * 0.025 L = 0.0025 mol
Therefore the molarity of HCl is:
Molarity HCl = 0.0025 mol / 0.025 L = 0.100 M
