Translation:Which chair is it?
「椅子」•「いす」 is a word for 'chair' or 'stool'. 「席」•「せき」is a word for 'seat' or 'place'
Additional to the breakdown provided above, those sentances have two slightly different meanings:
"Which chair is it" is an open question requesting additional information. E.g. There are several chairs and you have no current idea of which is specified.
"Is it this chair" is a closed question that has already formed a presumption and is requesting affirmation. E.g. You've already "chosen" a chair that might be the correct one whereas in the prior question you havent.
Pedantic difference, but present nevertheless.