How would you make 50 mL of a 0.100 M hydrochloric acid solution from concentrated (12.1 M) hydrochloric acid?

This is just a proportion question. First, you set it up as 50ml x 0.1M = Vml x 12.1M, where V stands for the volume of the concentrated acid. So V = (50 x 0.1)/12.1 = 0.41ml. You need to work in a fume hood and wear rubber/plastic gloves until the conc. acid is diluted with water. You would have to first measure out the 0.41ml of the conc. acid very carefully into a small graduated cylinder. (If the conc. acid is in a large container, pour some out into a beaker, then pour from the beaker into the grad. cylinder.) You will need to have a 50ml or 100ml graduated cylinder ready. Add about 30ml of water to this cylinder. Carefully pour the 0.41ml of acid into this cylinder, draining as much as possible, and let it partly mix by swirling it gently. Now add a few ml of water to the small grad. cylinder (where the acid was), swirl it a little, and pour it into the bigger cylinder. Repeat two or three times. Finally, fill the larger cylinder to a total volume of 50 ml. Pour this out into a beaker, which will completely mix the diluted acid. This is a very safe procedure to follow.