The Mali midfielder, making only his 10th league start for Pep Guardiola's side this season, swept home a deflected Lionel Messi cross in the 43rd minute at the Nou Camp to lift Barca to 74 points from 27 matches.
It was his fourth goal against Zaragoza in their last two visits to the Catalan capital following his hat-trick against the Aragonese club in a 6-1 home win in October 2009.
Zaragoza goalkeeper Toni Doblas made a string of brilliant saves to prevent a bigger margin of victory for Barca, who rested key players including David Villa, Andres Iniesta and Sergio Busquets with an eye on Tuesday's Champions League last-16, second leg at home to Arsenal.
"Now they [Real] have the pressure," Barca midfielder Xavi told reporters.
"We dominated and kept the ball and we are on a good run," the Spain World Cup-winner added.
"Now we just need to wait for Real to slip up. It's more and more difficult to win away from home."
In the earlier kick-off, Pablo Hernandez struck twice to give third-placed Valencia a 2-1 comeback win at Real Mallorca that put Unai Emery's side four points clear of Villarreal, who lost 3-1 at Atletico Madrid in the late game.
Former Arsenal winger Jose Antonio Reyes, Argentina forward Sergio Aguero and Uruguay striker Diego Forlan scored for the Europa League champions to lift them to sixth and boost their chances of a European berth next season.
Reyes set the Calderon alight with a cracking goal to give Atletico a fifth-minute lead, curling the ball into the top corner from 30 metres.
Giuseppe Rossi levelled for the visitors in the 34th before Aguero used his strength to hold off his marker and send a delicate lob over Diego Lopez to make it 2-1 in the 69th.
Forlan, who is having an indifferent season, ran on to a Reyes pass to make the game safe three minutes later.
Mallorca's Ivan Ramis converted a dubious penalty shortly after the half hour at their Iberostar stadium but the lead lasted only two minutes before Pablo drove home from just outside the area.
He then grabbed the winner for Unai Emery's side in the 56th minute when he ran on to a Jonas Goncalves pass and beat Dudu Aouate at his near post with an angled shot.
Mallorca, coached by former Denmark international Michael Laudrup, are 10th on 34, while Zaragoza remained in 15th on 27 points just two points above the drop zone.
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