空子 (kòng zi) is a noun meaning 'loophole' or 'gap' — a flaw or weakness in a system, rule, or situation that someone can take advantage of. It's most commonly used in the collocation 钻空子 (zuān kòngzi), 'to exploit a loophole'. You can also use it in more literal senses like 'a gap in time' (趁着开会中间的∼干点别的). The English glosses 'chance' and 'opportunity' are misleading if they imply a positive opportunity — 空子 usually carries a negative or opportunistic tone, referring to an opening someone shouldn't be taking.