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CARACAS, March 4 (Reuters) - Venezuela's Central Bank on Thursday launched a $50 million bond, the seventh such offering aimed at propping up the bolivar on a freely floated, semi-legal market since a currency devaluation in January.

The zero coupon bond is priced at 112 percent and works as a foreign exchange instrument because it can be purchased at an official rate of 4.3 bolivars to the dollar but will be paid in dollars after 90 days.

It can be traded on secondary markets within Venezuela.

President Hugo Chavez devalued the bolivar on Jan. 8, creating a two-tiered official system that sells dollars mostly for 4.3 bolivars, but allows some priority imports at 2.6.

The Central Bank issues are aimed at strengthening the bolivar on the separate freely-floated market. The currency has weakened in that mechanism during recent weeks to trade at around 6.6/6.8 on Thursday.

Thursday's offer was open for one hour between 12.30 p.m. and 1.30 p.m. local time, a bank statement said.

(Reporting by Eyanir Chinea, Writing by Andrew Cawthorne; Editing by Andrew Hay)