Question

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

Answer 1

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