The current cup holders were always in control of Sunday’s game at The Emirates from the moment Mertesacker headed home with 20 minutes on the clock.
Theo Walcott made his first start for nearly a year and should have doubled Arsenal???s lead in the second half but chipped an effort wide from just eight yards.
Hull huffed and puffed in their pursuit of an equaliser but Sanchez???s 16th goal of the season with eight minutes remaining ended the game as a contest.??
Arsenal started brightly and had Hull under the cosh from the first whistle. Joel Campbell had a glorious opportunity to opening the scoring when he was played through one-on-one with Steve Harper but his side-footed effort was too close to the Hull goalkeeper who cleared the ball away.
Sanchez was the next Arsenal player to come close to opening the scoring on 14 minutes. The returning Theo Walcott played in the Chilean but he couldn???t find a way through the Hull backline as his delicious chipped effort dropped agonizingly over the crossbar.
Arsenal were nearly caught out moments later when Tom Ince charged in from the right-hand side before firing a low effort just wide of David Ospina’s near post.
That miss proved costly as Arsenal were in front on 20 minutes.
Sanchez???s brilliant corner from the left eluded everyone but Mertesacker at the back post. The giant German showed more determination that Hull defender Curtis Davies and powered a header from four yards into the net.
The lead was nearly doubled just three minutes later by the brilliant Sanchez.??
The link-up between him and Tomas Rosicky was sublime as the tricky frontman darted in behind Davies. Sanchez showed fantastic reactions to jink his way around Harper but his goal-bound shot was somehow cleared brilliantly off the line by Paul McShane.??
The visitors grew in stature in the second period, keeping possession more fluently through the midfield but struggled to create any meaningful opportunities to the frustration of boss Steve Bruce.
In comparison, Arsenal looked threatening every time they ventured towards the Hull area.
Walcott should have marked his return from almost a year on the sidelines with a goal on 67 minutes when he was played through courtesy of a wonderful Santi Cazorla pass.
The England man???s first touch killed the ball into his path eight yards from but his finish lacked composure as he lifted the ball over Harper but horribly wide of the post.
Arsenal continued to press and Sanchez was denied by the Hull goalkeeper on 73 minutes after an out of character bad touch had allowed Harper to deny the striker from a tight angle.
The hosts did get a second goal that their play deserved with eight minutes left.
Sanchez received a wonderful pass from Cazorla on the edge of the Hull City area and a second later the ball was in the back of net as the mesmerising striker was given far too much space by Davies to turn and cooly fire the ball into the bottom corner.??
The Gunners have now lost just one of their last 39 FA Cup home matches.