Calculate the concentration of a NaOH solution if 20.0 mL of the solution is needed to neutralize 23.0 mL of a 0.312 M HCl solution?

No. of milli-moles of HCl = (0.312 mmol/mL) × (23.0 mL) = 7.18 mmol

NaOH + HCl → NaCl + H₂O
Mole ratio NaOH : HCl = 1 : 1
No. of milli-moles of NaOH = No. of milli-moles of HCl = 7.176 mmol

Concentration of NaOH = (7.18 mmol) / (20.0 mL) = 0.359 M