This is a simple acid base reaction.
To neutralize all NaOH, the moles in 4.00g of NaOH should be equal to the moles of HCl since the stoichiometric ratio between NaOH and HCl is 1 : 1.
NaOH + HCl → NaCl + H₂O
Molar mass of NaOH = (23 + 16 + 1 ) g mol⁻¹
Moles in NaOH = mass / molar mass = 4.00 g / 40 g mol⁻¹ = 0.1 mol
HCl moles = 0.1 mol
concentration = number of moles (mol) / volume (L)
Hence HCl volume = 1.0 mol/L x 0.1 mol = 0.1 L = 100.0 mL
To neutralize 4.00 g of NaOH, 100.0 mL of 1.0 M HCl is needed.
