What is the concentration of a 100L tank of HNO3 solution if 20.0ml of solution is removed from the tank...

and titrated with 0.125M Ca(OH)2. 5.20ml of titrant were needed to reach the end point.

2HNO3 + Ca(OH)2 ----------------> Ca(NO3)2 + 2 HOH
20 ml 0.125 mol/L
C = ? 5.20 mL

Find moles of Ca(OH)2 used. moles = C x V, = 0.125 mol/L x 5.20 mL = 0.65 millimoles

You need TWICE as many moles of HNO3 as you have Ca(OH)2. So the number of moles of HNO3 used will be 0.65 millimoles x 2 = 1.30

Now you have a Volume and a mole number for the HNO3

Concentration = n / V 1.30 millimol / 20.0 ml = 0.065 mol/L