Question: concentrated hcl is 37 by mass?
Answer: Compounding 1 liter of 0.1N Solution 12M (37% HCL) = 12 moles/L = 12 x 36.5 = 438 g/L = 438 mg/ml. 0.1 M x 36.5 = 3.65 g/L = 3650 mg.