A 0.105 L sample of an unknown HNO solution required 54.5ML of 0.100M Ba(OH)2 for complete neutralization. What is the concentration of the HNO3 solution

how many moles of Ba(OH)2 are there?

0.1mol/L * 0.0545L = 0.00545 moles
Ba(OH)2 + 2HNO3 → Ba(NO3)2 + 2H2O
So you will need 0.019 moles of HNO3
0.105L * xM = 0.019
x = 181mL