For some reason most-populous just doesn't sound right when used in a sentence. Most-populated makes more sense to me. Here is the sentence that it's used in for context.
“BLANK is the public health care system for the nation's third most-populous county.”
Any help on the usage of these 2 phrases would be much appreciated. Thank you in advance!
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I would think either would work. Note that populated and populous don't mean exactly thie same thing. Any place that has inhabitants at all is populated. Populous means heavily populated. Since you already say third most, you don't need to say populous, but you certainly could if you wanted to emphasize the large number of people (just like saying: "the third most heavily populated …") By the way, you shouldn't be using a hyphen. It's just "third most populated…" etc.
Populous is the correct word.
“Populous” is an adjective meaning “having a large population.” In your sentence it modifies the noun “county.”
“Populated” is an inflected form of the transitive verb “populate, which means “to furnish or provide with inhabitants.” Using it as an adjective is improper.
Your hyphenation is correct. “Most-populous” is a phrasal adjective; it modifies the noun as if one word.