Populous, meaning having many inhabitants, is always an adjective. Populace, meaning population or the general public, is always a noun. So, for example, a populous city has a large populace.
Populace vs. population?
While populace is synonymous with population, the two words’ connotations are slightly different. Population is neutral, while populace often carries a slightly superior tone toward the group it denotes. That’s why population is usually the better word choice.
Examples
These writers use populous and populace correctly:
Unfortunately, he’s forgotten how to speak German, and the populace can’t understand his heavily accented English. [Los Angeles Times]
The question now is whether it is capable of carrying the most populous Arab state toward new territory . . . [Wall Street Journal]
The country has undergone a traumatic period in the last three months with an absentee President and a restless populace. [All Africa]
The result leaves the city the second-most populous in the state, with a population behind Little Rock’s 193,524. [Times Record Online]
And these writers use the wrong word:
A well armed populous is a deterrent to Big Brother but not an insurmountable obstacle. [News With Views]
The most populace country in the Arab world has gone to the polls to vote . . . [Majalla Magazine]
