0.250 mol/L
Step 1. Write the chemical equation
H2SO4 + 2NaOH → Na2SO4 + 2H2O
Step 2. Calculate the moles of H2SO4
Moles of H2SO4 = 12.5 mL H2SO4 × (0.500 mmol H2SO4/1 mL H2SO4)
= 6.25 mmol H2SO4
Step 3. Calculate the moles of NaOH
Moles of NaOH = 6.25 mmol H2SO4 × (2 mmol NaOH/(1 mmol H2SO4)
= 12.5 mmol NaOH
Step 4. Calculate the concentration of the NaOH
[NaOH] = moles/litres = 12.5 mmol/50.0 mL = 0.250 mol/L