And so to gets the #"volume"#..we arrange the given quotient..
#"volume of solution"="moles of solute"/"concentration"#
#a.# And so if #3.0*mol# are required from #12.0*mol*L^-1# #HCl#...we take the quotient...
#(3.0*mol)/(12.0*mol*L^-1)=0.250*L# (mind you I don't think you can get #HCl# at this concentration....)
#b.# And if #3.0*mol# are required from #2.0*mol*L^-1# #HCl#...we take the quotient...
#(3.0*mol)/(2.0*mol*L^-1)=1.50*L#
#c.# And if #3.0*mol# are required from #0.5*mol*L^-1# #HCl#...we take the quotient...
#(3.0*mol)/(0.50*mol*L^-1)=6.0*L#
#d.# And if #3.0*mol# are required from #0.010*mol*L^-1# #HCl#...we take the quotient...
#(3.0*mol)/(0.010*mol*L^-1)=300.0*L#
And clearly this last is an impratical proposition....