Name the state class for  iowa admitted to the union december 28, 1846
CREATE TABLE "table1_225205_3" (
  "state_class" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_of_successors_formal_installation" text
);
SELECT "state_class" FROM "table1_225205_3" WHERE "reason_for_change"='Iowa admitted to the Union December 28, 1846';