Please open the page of 125 125쪽을 펼쳐 봐요. or 125 페이지를 펼쳐 봐요. 장 means "chapter" 125장을 열어 봐요. means "Pls open the chapter 125"

My teacher and I recently discussed this. I love when other languages have words for things I always wished existed in English! 쪽 : page-sides (see, awkward in English). Remember that 쪽 means side. So if you were suppose to read 4쪽, you would be reading pg. 1, 2, 3, 4. 장 : pages (each page in a book generally has two sides, or 2 쪽). So assuming each page has two sides, "read 4장" would mean reading pg. 1, 2, 3, 4, 5, 6, 7, 8. 1장 = 2쪽. If your teacher assigns an essay to write, you would hope the page requirements are in 쪽 not 장 because 쪽 is half as long.