Question

What is the amount of 4M HCl needed to completely neutralize 50mL of 2M NaOH

Answer 1

mols NaOH = M x L = 2 x 0.05 = 0.10
mols HCl must be 0.10
Then M HCl = mols HCl/L HCl.
L HCl = mols HCl/M HCl = 0.10/4M = ?

Answer 2

According to the equation HCl+NaOH = NaCl+H2O it will take just as many moles of HCl as NaOH to do the job.

So, now just do the math. If it will take x mL, then

4x = 2*50