30.0 mL of 0.50 M NaOH to neutralize 15.0 mL of HNO3, what is the concentration (molarity) of the HNO3?

First find the no. of moles of NaOH:

30/1000 = 0.3 dm3 so no. of moles = 0.3*0.5 = 0.15 moles

as NaOH reacts with HNO3 in a ratio of one to one, there must have been 0.15 moles of HNO3 too

moles/volume = concentration

volume = 15/1000 = 0.15 dm3

concentration = 1.15/0.15 = 1 mol. dm-3

The quicker way would be to realize that you used twice as much NaOH so the HNO3 had to be twice as strong