CREATE TABLE table_202_158 (id DECIMAL, "z" DECIMAL, "element" TEXT, "observed or predicted" TEXT, "isolated  ( widely known ) " DECIMAL, "observer" TEXT, "first isolator" TEXT, "notes" TEXT)

which element was observed or predicted in 1771 by w.scheele ?
SELECT "element" FROM table_202_158 WHERE "observed or predicted" = 1771 AND "observer" = 'w. scheele'
SELECT "element" FROM "table_202_158" WHERE "observed or predicted" = 1771 AND "observer" = 'w. scheele'