[Josette017]

How do you calculate the number of moles of HNO₃ reacting when 50.0 mL of 1.00M HNO₃ and 50.5 mL of 1.00M NaOH solution are mixed.

Thank you for your help.

[AWK]

Use a definition of molar concentration - moles in 1 liter of solution. This means that moles are calculated by multiplying the molar concentration of solution by a volume in liters.
Your question concnerns moles before neutralization since your problem has an excess of NaOH. After moxing no HNO3 will be present in solution.