Bryan Bickell also scored and Patrick Sharp added a pair of helpers for the Blackhawks, who have won back-to-back games since dropping three straight.
Milan Michalek scored and Robin Lehner gave up both goals on 39 shots for the Senators, who had won six of their last seven games coming in.
Chicago took the lead for good at 9:48 of the second period on the power play after Hossa collected the puck down low to the left of the net. Hossa brought the puck around behind and stuffed home the wrap-around attempt to make it 2-1.
The Senators pulled Lehner with around a minute to play in favor of the extra attacker. Ottawa got some shots on net, but was ultimately unable to find the tying goal.
The Senators struck first with 4:25 left in the first period after Jason Spezza weaved his way through the neutral zone and gained the blue line on the right wing. Spezza looked up and sent a pass into the slot for Michalek, who fired a wrister that beat Emery low to the stick side.
Chicago answered at 5:14 of the second period on a delayed penalty. Skating with a 6-on-5 advantage, the Blackhawks kept control of the puck in the offensive zone and Marcus Kruger took the puck behind the Ottawa net, reversed it to Sharp along the goal line and Sharp then fed Bickell, who roofed it from in close.
Ottawa failed to score on its only power play chance, while the Blackhawks went 1-for-2 with the man advantage...The Blackhawks will visit the Detroit Red Wings on Sunday, while the Senators will visit the Florida Panthers also on Sunday.