Hyphens, En Dashes, Em Dashes

Q. There is one type of compound adjective that has been causing me grief, and I was hoping you might be able to clarify it for me: “high–molecular weight hydrocarbon” or “high-molecular-weight hydrocarbon” or “high molecular weight hydrocarbon”? The sentence, for context: Following flocculation using a high-molecular-weight, medium-charge density polyacrylamide flocculent at optimal dosage, the filterability of flocculated sediments from tailings containing an intermediate fines content of solids from 10 to 20 wt. % was improved drastically. Thank you kindly!

A. In my experience, flocculating almost always leads to trouble. Although your version with the en dash is technically correct, it doesn’t make clear how many of the words following are included in the compound. I would write instead “high-molecular-weight hydrocarbon” to clarify what goes with what, just as you did by writing “medium-charge density polyacrylamide flocculent” (lest we misread “medium charge-density”).