By using the following formula we can calculate the volume; C1V1 = C2V2 let V1 is the volume we have to find, then C1 = 12.1m C2 = 0.250m V2 = 5 l Now, C1V1 = C2V2 12.1 x V1 = 0.250 x 5 V1 = 1.25 / 12.1 V1 = 0.1033 l thus, the 0.1033 liter of HCl is necessary.