"X 있잖아요" is an idiomatic expression, similar to, "Why, you know, X." (Literally, it means "isn't there X?" - from which it's easy to understand how this means "you know".) For example,
A: 찬장에서 밀가루 좀 꺼내 줄래? = Can you pass me the flour from the cupboard?
B: 어느 게 밀가루야? = Which one is flour?
A: 파란 종이 봉지에 들어있는 거 있잖아. = You see, the one in the blue paper bag.
On the other hand, "X(이)잖아요" literally means "isn't it X?", so you could use it in a different situation. For example:
B: 부침개에 밀가루를 그렇게 많이 넣으려고? = You are pouring so much flour on the pancake (mix)?
A: 왜? 부침개가 원래 밀가루 맛으로 먹는 거잖아? = Why (not)? Pancakes are supposed to have floury taste, isn't it?
A note of caution: when you connect 거 with -이잖아 (or any other form of the copula -이다), the 이 disappears, so it must be 거다/거냐/거잖아, not 거이다/거이냐/거이잖아. (거 is already an informal form of 것, so adding the full -이다 after that is impossible, as far as I know.)