The key to any dilution calculation is the dilution factor
#color(blue)(ul(color(black)("DF" = V_"diluted"/V_"stock" = c_"stock"/c_"diluted")))#
The dilution factor essentially tells you how concentrated the stock solution was compared with the diluted solution.
In your case, the dilution must take you from a concentrated hydrochloric acid solution of
#"DF" = (18.5 color(red)(cancel(color(black)("M"))))/(1.5color(red)(cancel(color(black)("M")))) = color(blue)(12.333)#
So, in order to decrease the concentration of the stock solution by a factor of
The volume of the stock solution needed for this dilution will be
#"DF" = V_"diluted"/V_"stock" implies V_"stock" = V_"diluted"/"DF"#
Plug in your values to find
#V_"stock" = "25.0 L"/color(blue)(12.333) = color(darkgreen)(ul(color(black)("2.0 L")))#
So, to make
IMPORTANT NOTE! Do not forget that you must always add concentrated acid to water and not the other way around!
In this case, you're working with very concentrated hydrochloric acid, so it would be best to keep the stock solution and the water needed for the dilution in an ice bath before the dilution.
Also, it would be best to perform the dilution in several steps using smaller doses of stock solution. Don't forget to stir as you're adding the acid!
So, to dilute your solution, take several steps to add the concentrated acid solution to enough water to ensure that the final is as close to
#"Water to concentrated acid " -> color(white)(.)color(red)("NO!")#
#"Concentrated acid to water " -> color(white)(.)color(darkgreen)("YES!")#