Representative Trey Radel, Republican of Florida, pleaded guilty on Wednesday to a misdemeanor charge stemming from his purchase of cocaine in Washington last month. He was sentenced to one year of probation. According to documents released by prosecutors, federal authorities learned this fall that Mr. Radel, a freshman legislator, had purchased cocaine on several occasions for his own use in Washington and would sometimes share it with others. As part of a sting operation last month, an undercover police officer and an associate of Mr. Radel’s met him at a restaurant near Dupont Circle, and Mr. Radel invited them back to his apartment to use cocaine. After they declined, the officer told Radel that he had cocaine to sell.