This is a titration problem where acid is allowed to be neutralized by a base. The chemical equation for this reaction is

 HNO3 + NaOH --> H2O + NaNO3

To get the value of the concentration of the nitric acid solution we have to use the amount of NaOH solution used in the reaction.

In neutralization reaction:

Moles acid = moles base

Therefore:

Moles NaOH = moles HNO3

Remember that, Molarity = moles/volume(L)

So, moles = Molarity * Volume (L)

(Molarity x volume)for NaOH = (Molarity x Volume) for HNO3

Substituting the value of the given:

0.100 moles/L*(12.5/1000)L = Molarity*(50/1000)L

Then we solve for the Molarity of HNO3

Molarity of HNO3 = (0.100x 0.0125)/(0.05)

Molarity of HNO3 = 0.025 moles/L