We use the definition of concentration......
#"Concentration"="Moles of solute"/"Volume of solution"#
Now we want #500*mLxx10^-3*L*mL^-1xx4*mol*L^-1#. Agreed?
And thus here the moles of solute ...........#500*cancel(mL)xx10^-3*L*cancel(mL^-1)xx4*mol*cancel(L^-1)=2*mol#.
We have #16*mol*L^-1# #HCl# available.....and so from the equation above....
#"Volume"="Mole of solute"/"Concentration"=(2*mol)/(16*mol*L^-1)=1/8*L#
#=0.125*L#
Just to add, that this question is chemically unrealistic. The strongest hydrochloric acid you can buy commercially is approx. #"34%(w/w)"#; this has a molar concentration of #12*mol*L^-1#. And practically, when you mix acid and water, you ALWAYS add [acid to water.](https://socratic.org/questions/in-what-order-should-strong-acids-and-water-be-mixed?source=search)
